Critique of Pure Reason

(Prefaces and Introduction)

P 004
De nobis ipsis silemus:  De re autem, quae agitur, petimus: ut
homines eam non Opinionem, sed Opus esse cogitent; ac pro certo
habeant, non Sectae nos alicujus, aut Placiti, sed utilitatis et
amplitudinis humanae fundamenta moliri. Deinde ut suis commodis 
aequi ... in commune consulant, ... et ipsi in partem veniant. Praeterea
ut bene sperent, neque Instaurationem nostram ut quiddam infini-
tum et ultra mortale fingant, et animo concipiant; quum revera sit
infiniti erroris finis et terminus legitimus. 
P 005
To his Excellency
The Royal Minister of State
Baron von Zedlitz 
 To further, so far as in us lies, the growth of the sciences is
to work along the lines of your Excellency's own interests, which
are closely bound up with the sciences, not only in virtue of your
exalted position as a patron, but through your more intimate
relation to them as lover and enlightened judge. I therefore avail
myself of the only means that is in any degree in my power, of
expressing my gratitude for the gracious confidence with which your
Excellency honours me, if that I could perhaps be of assistance in
this respect. 
 To the same gracious attention with which your Excellency has
honoured the first edition of this work I now dedicate this second
edition, and therewith I crave the protection of all the other con-
cerns of my literary mission, and remain with the most profound
Your Excellency's
Humble, most obedient servant,
Whoever limiting his worldly ambitions finds satisfaction in the speculative
life has in the approval of an enlightened and competent judge a powerful
incentive to labours, the benefits of which are great but remote, and
therefore such as the vulgar altogether fail to recognise. 
 To such a judge and to his gracious attention I now dedicate this work, and
to his 
HUMAN reason has this peculiar fate that in one species
of its knowledge it is burdened by questions which, as pre-
scribed by the very nature of reason itself, it is not able to
ignore, but which, as transcending all its powers, it is also
not able to answer. 
 The perplexity into which it thus falls is not due to any
fault of its own. It begins with principles which it has no
option save to employ in the course of experience, and which
this experience at the same time abundantly justifies it in
using. Rising with their aid (since it is determined to this
also by its own nature) to ever higher, ever more remote,
conditions, it soon becomes aware that in this way -- the
questions never ceasing -- its work must always remain
incomplete; and it therefore finds itself compelled to resort
to principles which overstep all possible empirical employ-
ment, and which yet seem so unobjectionable that even
ordinary consciousness readily accepts them. But by this
procedure human reason precipitates itself into darkness
and contradictions; and while it may indeed conjecture
that these must be in some way due to concealed errors,
it is not in a position to be able to detect them. For since
the principles of which it is making use transcend the limits
of experience, they are no longer subject to any empirical
test. The battle-field of these endless controversies is called
 Time was when metaphysics was entitled the Queen of
all the sciences; and if the will be taken for the deed, the pre-
eminent importance of her accepted tasks gives her every
right to this title of honour. Now, however, the changed
fashion of the time brings her only scorn; a matron outcast
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and forsaken, she mourns like Hecuba: Modo maxima rerum,
tot generis natisque potens -- nunc trahor exul, inops. 
Her government, under the administration of the dogmat-
ists, was at first despotic. But inasmuch as the legislation
still bore traces of the ancient barbarism, her empire gradu-
ally through intestine wars gave way to complete anarchy;
and the sceptics, a species of nomads, despising all settled
modes of life, broke up from time to time all civil society. 
Happily they were few in number, and were unable to prevent
its being established ever anew, although on no uniform and
self-consistent plan. In more recent times, it has seemed as
if an end might be put to all these controversies and the
claims of metaphysics receive final judgment, through a
certain physiology of the human understanding -- that of the
celebrated Locke. But it has turned out quite otherwise. For
however the attempt be made to cast doubt upon the pre-
tensions of the supposed Queen by tracing her lineage to
vulgar origins in common experience, this genealogy has,
as a matter of fact, been fictitiously invented, and she has
still continued to uphold her claims. Metaphysics has accord-
ingly lapsed back into the ancient time-worn dogmatism, and
so again suffers that depreciation from which it was to have
been rescued. And now, after all methods, so it is believed,
have been tried and found wanting, the prevailing mood is
that of weariness and complete indifferentism -- the mother,
in all sciences, of chaos and night, but happily in this case
the source, or at least the prelude, of their approaching
reform and restoration. For it at least puts an end to that ill-
applied industry which has rendered them thus dark, confused,
and unserviceable. 
 But it is idle to feign indifference to such enquiries,
the object of which can never be indifferent to our human
nature. Indeed these pretended indifferentists, however
they may try to disguise themselves by substituting a
popular tone for the language of the Schools, inevitably
fall back, in so far as they think at all, into those very
metaphysical assertions which they profess so greatly to
 Ovid, Metam. 
P 009
None the less this indifference, showing itself in the
midst of flourishing sciences, and affecting precisely those
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sciences, the knowledge of which, if attainable, we should
least of all care to dispense with, is a phenomenon that
calls for attention and reflection. It is obviously the effect
not of levity but of the matured judgment of the age, which
refuses to be any longer put off with illusory knowledge. It is
a call to reason to undertake anew the most difficult of all
its tasks, namely, that of self-knowledge, and to institute
a tribunal which will assure to reason its lawful claims, and
dismiss all groundless pretensions, not by despotic decrees,
but in accordance with its own eternal and unalterable
laws. This tribunal is no other than the critique of pure
 I do not mean by this a critique of books and systems,
but of the faculty of reason in general, in respect of all know-
ledge after which it may strive independently of all experi-
ence. It will therefore decide as to the possibility or impossi-
bility of metaphysics in general, and determine its sources,
its extent, and its limits -- all in accordance with principles. 
 I have entered upon this path -- the only one that has re-
mained unexplored -- and flatter myself that in following it I
have found a way of guarding against all those errors which
have hitherto set reason, in its non-empirical employment, at
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variance with itself. 
P 009n
 We often hear complaints of shallowness of thought in our age
and of the consequent decline of sound science. But I do not see
that the sciences which rest upon a secure foundation, such as mathe-
matics, physics, etc. , in the least deserve this reproach. On the con-
trary, they merit their old reputation for solidity, and, in the case
of physics, even surpass it. The same spirit would have become
active in other kinds of knowledge, if only attention had first been
directed to the determination of their principles. Till this is done, in-
difference, doubt, and, in the final issue, severe criticism, are them-
selves proofs of a profound habit of thought. Our age is, in especial
degree, the age of criticism, and to criticism everything must sub-
mit. Religion through its sanctity, and law-giving through its majesty,
may seek to exempt themselves from it. But they then awaken just
suspicion, and cannot claim the sincere respect which reason accords
only to that which has been able to sustain the test of free and open
P 010
I have not evaded its questions by plead-
ing the insufficiency of human reason. On the contrary, I have
specified these questions exhaustively, according to prin-
ciples; and after locating the point at which, through mis-
understanding, reason comes into conflict with itself, I have
solved them to its complete satisfaction. The answer to these
questions has not, indeed, been such as a dogmatic and vision-
ary insistence upon knowledge might lead us to expect --
that can be catered for only through magical devices, in which
I am no adept. Such ways of answering them are, indeed, not
within the intention of the natural constitution of our reason;
and inasmuch as they have their source in misunderstanding,
it is the duty of philosophy to counteract their deceptive in-
fluence, no matter what prized and cherished dreams may have
to be disowned. In this enquiry I have made completeness
my chief aim, and I venture to assert that there is not a single
metaphysical problem which has not been solved, or for the
solution of which the key at least has not been supplied. Pure
reason is, indeed, so perfect a unity that if its principle were
insufficient for the solution of even a single one of all the
questions to which it itself gives birth we should have no
alternative but to reject the principle, since we should then no
longer be able to place implicit reliance upon it in dealing
with any one of the other questions. 
 While I am saying this I can fancy that I detect in the face
of the reader an expression of indignation, mingled with con-
tempt, at pretensions seemingly so arrogant and vain-glorious. 
Yet they are incomparably more moderate than the claims
of all those writers who on the lines of the usual programme
profess to prove the simple nature of the soul or the necessity
of a first beginning of the world. For while such writers pledge
themselves to extend human knowledge beyond all limits of
possible experience, I humbly confess that this is entirely be-
yond my power. I have to deal with nothing save reason itself
and its pure thinking; and to obtain complete knowledge of
these, there is no need to go far afield, since I come upon them
in my own self. Common logic itself supplies an example, how
all the simple acts of reason can be enumerated completely
and systematically. The subject of the present enquiry is the
[kindred] question, how much we can hope to achieve by
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reason, when all the material and assistance of experience are
taken away. 
 So much as regards completeness in our determination of
each question, and exhaustiveness in our determination of all
the questions with which we have to deal. These questions are
not arbitrarily selected; they are prescribed to us, by the very
nature of knowledge itself, as being the subject-matter of our
critical enquiry. 
 As regards the form of our enquiry, certainty and clearness
are two essential requirements, rightly to be exacted from any-
one who ventures upon so delicate an undertaking. 
 As to certainty, I have prescribed to myself the maxim,
that in this kind of investigation it is in no wise permissible to
hold opinions. Everything, therefore, which bears any manner
of resemblance to an hypothesis is to be treated as contra-
band; it is not to be put up for sale even at the lowest price,
but forthwith confiscated, immediately upon detection. Any
knowledge that professes to hold a priori lays claim to be
regarded as absolutely necessary. This applies still more to any
determination of all pure a priori knowledge, since such deter-
mination has to serve as the measure, and therefore as the
[supreme] example, of all apodeictic (philosophical) certainty. 
Whether I have succeeded in what I have undertaken must be
left altogether to the reader's judgment; the author's task is
solely to adduce grounds, not to speak as to the effect which
they should have upon those who are sitting in judgment. But
the author, in order that he may not himself, innocently, be
the cause of any weakening of his arguments, may be permitted
to draw attention to certain passages, which, although merely
incidental, may yet occasion some mistrust. Such timely inter-
vention may serve to counteract the influence which even quite
undefined doubts as to these minor matters might otherwise
exercise upon the reader's attitude in regard to the main
 I know no enquiries which are more important for ex-
ploring the faculty which we entitle understanding, and for
determining the rules and limits of its employment, than those
which I have instituted in the second chapter of the Trans-
cendental Analytic under the title Deduction of the Pure
Concepts of Understanding. They are also those which have
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cost me the greatest labour -- labour, as I hope, not unre-
warded. This enquiry, which is somewhat deeply grounded,
has two sides. The one refers to the objects of pure under-
standing, and is intended to expound and render intelligible
the objective validity of its a priori concepts. It is therefore
essential to my purposes. The other seeks to investigate the
pure understanding itself, its possibility and the cognitive
faculties upon which it rests; and so deals with it in its sub-
jective aspect. Although this latter exposition is of great
importance for my chief purpose, it does not form an essential
part of it. For the chief question is always simply this: -- what
and how much can the understanding and reason know apart
from all experience? not: -- how is the faculty of thought itself
possible? The latter is, as it were, the search for the cause of
a given effect, and to that extent is somewhat hypothetical
in character (though, as I shall show elsewhere, it is not really
so); and I would appear to be taking the liberty simply of
expressing an opinion, in which case the reader would be free
to express a different opinion. For this reason I must forestall
the reader's criticism by pointing out that the objective de-
duction with which I am here chiefly concerned retains its full
force even if my subjective deduction should fail to produce
that complete conviction for which I hope. On this matter,
what has been said on pp. 92-93 should in any case suffice
by itself. 
 As regards clearness, the reader has a right to demand, in
the first place, a discursive (logical) clearness, through con-
cepts, and secondly, an intuitive (aesthetic) clearness, through
intuitions, that is, through examples and other concrete
illustrations. For the first I have sufficiently provided. That
was essential to my purpose; but it has also been the incidental
cause of my not being in a position to do justice to the second
demand, which, if not so pressing, is yet still quite reasonable. 
I have been almost continuously at a loss, during the progress
of my work, how I should proceed in this matter. Examples
and illustrations seemed always to be necessary, and so took
their place, as required, in my first draft. But I very soon
became aware of the magnitude of my task and of the multi-
plicity of matters with which I should have to deal; and as
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I perceived that even if treated in dry, purely scholastic
fashion, the outcome would by itself be already quite suffi-
ciently large in bulk, I found it inadvisable to enlarge it yet
further through examples and illustrations. These are neces-
sary only from a popular point of view; and this work can
never be made suitable for popular consumption. Such
assistance is not required by genuine students of the science,
and, though always pleasing, might very well in this case
have been self-defeating in its effects. Abbot Terrasson has
remarked that if the size of a volume be measured not by the
number of its pages but by the time required for mastering it,
it can be said of many a book, that it would be much shorter
if it were not so short. On the other hand, if we have in view
the comprehensibility of a whole of speculative knowledge,
which, though wide-ranging, has the coherence that follows
from unity of principle, we can say with equal justice that
many a book would have been much clearer if it had not made
such an effort to be clear. For the aids to clearness, though
they may be of assistance in regard to details, often interfere
with our grasp of the whole. The reader is not allowed to
arrive sufficiently quickly at a conspectus of the whole; the
bright colouring of the illustrative material intervenes to cover
over and conceal the articulation and organisation of the
system, which, if we are to be able to judge of its unity and
solidity, are what chiefly concern us. 
 The reader, I should judge, will feel it to be no small
inducement to yield his willing co-operation, when the author
is thus endeavouring, according to the plan here proposed, to
carry through a large and important work in a complete and
lasting manner. Metaphysics, on the view which we are adopt-
ing, is the only one of all the sciences which dare promise
that through a small but concentrated effort it will attain,
and this in a short time, such completion as will leave no
task to our successors save that of adapting it in a didactic
manner according to their own preferences, without their
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being able to add anything whatsoever to its content. For it
is nothing but the inventory of all our possessions through
pure reason, systematically arranged. In this field nothing
can escape us. What reason produces entirely out of itself
cannot be concealed, but is brought to light by reason itself
immediately the common principle has been discovered. 
The complete unity of this kind of knowledge, and the fact
that it is derived solely from pure concepts, entirely unin-
fluenced by any experience or by special intuition, such as
might lead to any determinate experience that would enlarge
and increase it, make this unconditioned completeness not
only practicable but also necessary. Tecum habita, et noris
quam sit tibi curta supellex. 
 Such a system of pure (speculative) reason I hope myself
to produce under the title Metaphysics of Nature. It will be
not half as large, yet incomparably richer in content than this
present Critique, which has as its first task to discover the
sources and conditions of the possibility of such criticism,
clearing, as it were, and levelling what has hitherto been waste-
ground. In this present enterprise I look to my reader for the
patience and impartiality of a judge; whereas in the other I
shall look for the benevolent assistance of a fellow-worker. 
For however completely all the principles of the system are
presented in this Critique, the completeness of the system
itself likewise requires that none of the derivative concepts
be lacking. These cannot be enumerated by any a priori com-
putation, but must be discovered gradually. Whereas, there-
fore, in this Critique the entire synthesis of the concepts has
been exhausted, there will still remain the further work of
making their analysis similarly complete, a task which is
rather an amusement than a labour. 
 I have only a few remarks to add of a typographical
character. As the beginning of the printing was delayed, I
was not able to see more than about half of the proof-sheets,
and I now find some misprints, which do not, however, affect
the sense except on p. 379, line 4 from the bottom, where
specific has to be read in place of sceptical. 
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The antinomy
P 015
of pure reason, from p. 425 to p. 461, has been so arranged,
in tabular form, that all that belongs to the thesis stands
on the left and what belongs to the antithesis on the right. 
This I have done in order that proposition and counter-
proposition may be the more easily compared with one
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WHETHER the treatment of such knowledge as lies within the
province of reason does or does not follow the secure path of a
science, is easily to be determined from the outcome. For if
after elaborate preparations, frequently renewed, it is brought
to a stop immediately it nears its goal; if often it is com-
pelled to retrace its steps and strike into some new line of
approach; or again, if the various participants are unable to
agree in any common plan of procedure, then we may rest
assured that it is very far from having entered upon the secure
path of a science, and is indeed a merely random groping. In
these circumstances, we shall be rendering a service to reason
should we succeed in discovering the path upon which it can
securely travel, even if, as a result of so doing, much that is
comprised in our original aims, adopted without reflection,
may have to be abandoned as fruitless. 
That logic has already, from the earliest times, proceeded
upon this sure path is evidenced by the fact that since Aris-
totle it has not required to retrace a single step, unless, indeed,
we care to count as improvements the removal of certain need-
less subtleties or the clearer exposition of its recognised teach-
ing, features which concern the elegance rather than the cer-
tainty of the science. It is remarkable also that to the present
day this logic has not been able to advance a single step, and
is thus to all appearance a closed and completed body of doc-
trine. If some of the moderns have thought to enlarge it by
introducing psychological chapters on the different faculties of
knowledge (imagination, wit, etc. ), metaphysical chapters on
the origin of knowledge or on the different kinds of certainty
according to difference in the objects (idealism, scepticism, etc. ),
or anthropological chapters on prejudices, their causes and
remedies, this could only arise from their ignorance of the
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peculiar nature of logical science. We do not enlarge but
disfigure sciences, if we allow them to trespass upon one
another's territory. The sphere of logic is quite precisely de-
limited; its sole concern is to give an exhaustive exposition and
a strict proof of the formal rules of all thought, whether it be
a priori or empirical, whatever be its origin or its object, and
whatever hindrances, accidental or natural, it may encounter
in our minds. 
That logic should have been thus successful is an advan-
tage which it owes entirely to its limitations, whereby it is
justified in abstracting -- indeed, it is under obligation to do
so -- from all objects of knowledge and their differences, leav-
ing the understanding nothing to deal with save itself and its
form. But for reason to enter on the sure path of science is,
of course, much more difficult, since it has to deal not with
itself alone but also with objects. Logic, therefore, as a pro-
paedeutic, forms, as it were, only the vestibule of the sciences;
and when we are concerned with specific modes of know-
ledge, while logic is indeed presupposed in any critical
estimate of them, yet for the actual acquiring of them we
have to look to the sciences properly and objectively so
Now if reason is to be a factor in these sciences, something
in them must be known a priori, and this knowledge may be
related to its object in one or other of two ways, either as
merely determining it and its concept (which must be supplied
from elsewhere) or as also making it actual. The former is
theoretical, the latter practical knowledge of reason. In both,
that part in which reason determines its object completely
a priori, namely, the pure part -- however much or little this part
may contain -- must be first and separately dealt with, in case
it be confounded with what comes from other sources. For it
is bad management if we blindly pay out what comes in, and
are not able, when the income falls into arrears, to distinguish
which part of it can justify expenditure, and in which line we
must make reductions. 
Mathematics and physics, the two sciences in which reason
yields theoretical knowledge, have to determine their objects
a priori, the former doing so quite purely, the latter having
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to reckon, at least partially, with sources of knowledge other
than reason. 
In the earliest times to which the history of human reason
extends, mathematics, among that wonderful people, the
Greeks, had already entered upon the sure path of science. But
it must not be supposed that it was as easy for mathematics as
it was for logic -- in which reason has to deal with itself alone --
to light upon, or rather to construct for itself, that royal road. 
On the contrary, I believe that it long remained, especially
among the Egyptians, in the groping stage, and that the trans-
formation must have been due to a revolution brought about
by the happy thought of a single man, the experiment which
he devised marking out the path upon which the science must
enter, and by following which, secure progress throughout all
time and in endless expansion is infallibly secured. The his-
tory of this intellectual revolution -- far more important than
the discovery of the passage round the celebrated Cape of
Good Hope -- and of its fortunate author, has not been pre-
served. But the fact that Diogenes Laertius, in handing down
an account of these matters, names the reputed author of even
the least important among the geometrical demonstrations,
even of those which, for ordinary consciousness, stand in need
of no such proof, does at least show that the memory of the
revolution, brought about by the first glimpse of this new path,
must have seemed to mathematicians of such outstanding im-
portance as to cause it to survive the tide of oblivion. A new light
flashed upon the mind of the first man (be he Thales or some
other) who demonstrated the properties of the isosceles triangle. 
The true method, so he found, was not to inspect what he dis-
cerned either in the figure, or in the bare concept of it, and from
this, as it were, to read off its properties; but to bring out what
was necessarily implied in the concepts that he had himself
formed a priori, and had put into the figure in the construction
by which he presented it to himself. If he is to know anything
with a priori certainty he must not ascribe to the figure any-
thing save what necessarily follows from what he has himself
set into it in accordance with his concept. 
Natural science was very much longer in entering upon the
highway of science. It is, indeed, only about a century and a
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half since Bacon, by his ingenious proposals, partly initiated
this discovery, partly inspired fresh vigour in those who were
already on the way to it. In this case also the discovery can
be explained as being the sudden outcome of an intellectual
revolution. In my present remarks I am referring to natural
science only in so far as it is founded on empirical principles. 
When Galileo caused balls, the weights of which he had
himself previously determined, to roll down an inclined plane;
when Torricelli made the air carry a weight which he had cal-
culated beforehand to be equal to that of a definite column of
water; or in more recent times, when Stahl changed metal
into lime, and lime back into metal, by withdrawing some-
thing and then restoring it, a light broke upon all students of
nature. They learned that reason has insight only into that
which it produces after a plan of its own, and that it must not
allow itself to be kept, as it were, in nature's leading-strings,
but must itself show the way with principles of judgment based
upon fixed laws, constraining nature to give answer to ques-
tions of reason's own determining. Accidental observations,
made in obedience to no previously thought-out plan, can
never be made to yield a necessary law, which alone reason is
concerned to discover. Reason, holding in one hand its prin-
ciples, according to which alone concordant appearances can
be admitted as equivalent to laws, and in the other hand the
experiment which it has devised in conformity with these prin-
ciples, must approach nature in order to be taught by it. It
must not, however, do so in the character of a pupil who
listens to everything that the teacher chooses to say, but of
an appointed judge who compels the witnesses to answer
questions which he has himself formulated. Even physics,
therefore, owes the beneficent revolution in its point of view
entirely to the happy thought, that while reason must seek in
nature, not fictitiously ascribe to it, whatever as not being
knowable through reason's own resources has to be learnt,
if learnt at all, only from nature, it must adopt as its guide,
in so seeking, that which it has itself put into nature. It is thus
that the study of nature has entered on the secure path of a
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science, after having for so many centuries been nothing but
a process of merely random groping. 
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 I am not, in my choice of examples, tracing the exact course of
the history of the experimental method; we have indeed no very pre-
cise knowledge of its first beginnings. 
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Metaphysics is a completely isolated speculative science of
reason, which soars far above the teachings of experience, and
in which reason is indeed meant to be its own pupil. Meta-
physics rests on concepts alone -- not, like mathematics, on their
application to intuition. But though it is older than all other
sciences, and would survive even if all the rest were swallowed
up in the abyss of an all-destroying barbarism, it has not yet
had the good fortune to enter upon the secure path of a science. 
For in it reason is perpetually being brought to a stand, even
when the laws into which it is seeking to have, as it professes,
an a priori insight are those that are confirmed by our most com-
mon experiences. Ever and again we have to retrace our steps,
as not leading us in the direction in which we desire to go. So
far, too, are the students of metaphysics from exhibiting any
kind of unanimity in their contentions, that metaphysics has
rather to be regarded as a battle-ground quite peculiarly suited
for those who desire to exercise themselves in mock combats,
and in which no participant has ever yet succeeded in gaining
even so much as an inch of territory, not at least in such
manner as to secure him in its permanent possession. This
shows, beyond all questioning, that the procedure of meta-
physics has hitherto been a merely random groping, and,
what is worst of all, a groping among mere concepts. 
What, then, is the reason why, in this field, the sure road
to science has not hitherto been found? Is it, perhaps, im-
possible of discovery? Why, in that case, should nature have
visited our reason with the restless endeavour whereby it is
ever searching for such a path, as if this were one of its most
important concerns. Nay, more, how little cause have we to
place trust in our reason, if, in one of the most important
domains of which we would fain have knowledge, it does
not merely fail us, but lures us on by deceitful promises, and
in the end betrays us! Or if it be only that we have thus far
failed to find the true path, are there any indications to justify
the hope that by renewed efforts we may have better fortune
than has fallen to our predecessors? 
The examples of mathematics and natural science, which
by a single and sudden revolution have become what they
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now are, seem to me sufficiently remarkable to suggest our
considering what may have been the essential features in the
changed point of view by which they have so greatly bene-
fited. Their success should incline us, at least by way of experi-
ment, to imitate their procedure, so far as the analogy which,
as species of rational knowledge, they bear to metaphysics may
permit. Hitherto it has been assumed that all our knowledge
must conform to objects. But all attempts to extend our know-
ledge of objects by establishing something in regard to them
a priori, by means of concepts, have, on this assumption,
ended in failure. We must therefore make trial whether we
may not have more success in the tasks of metaphysics, if
we suppose that objects must conform to our knowledge. This
would agree better with what is desired, namely, that it should
be possible to have knowledge of objects a priori, determining
something in regard to them prior to their being given. We
should then be proceeding precisely on the lines of Copernicus'
primary hypothesis. Failing of satisfactory progress in ex-
plaining the movements of the heavenly bodies on the supposi-
tion that they all revolved round the spectator, he tried whether
he might not have better success if he made the spectator
to revolve and the stars to remain at rest. A similar experi-
ment can be tried in metaphysics, as regards the intuition
of objects. If intuition must conform to the constitution of
the objects, I do not see how we could know anything of
the latter a priori; but if the object (as object of the senses)
must conform to the constitution of our faculty of intuition,
I have no difficulty in conceiving such a possibility. Since I
cannot rest in these intuitions if they are to become known,
but must relate them as representations to something as their
object, and determine this latter through them, either I must
assume that the concepts, by means of which I obtain this
determination, conform to the object, or else I assume that the
objects, or what is the same thing, that the experience in
which alone, as given objects, they can be known, conform to
the concepts. In the former case, I am again in the same per-
plexity as to how I can know anything a priori in regard to
the objects. In the latter case the outlook is more hopeful. For
experience is itself a species of knowledge which involves
P 023
understanding; and understanding has rules which I must pre-
suppose as being in me prior to objects being given to me, and
therefore as being a priori. They find expression in a priori
concepts to which all objects of experience necessarily con-
form, and with which they must agree. As regards objects
which are thought solely through reason, and indeed as
necessary, but which can never -- at least not in the manner
in which reason thinks them -- be given in experience, the
attempts at thinking them (for they must admit of being
thought) will furnish an excellent touchstone of what we are
adopting as our new method of thought, namely, that we can
know a priori of things only what we ourselves put into them. 
This experiment succeeds as well as could be desired, and
promises to metaphysics, in its first part -- the part that is
occupied with those concepts a priori to which the correspond-
ing objects, commensurate with them, can be given in ex-
perience -- the secure path of a science. For the new point of
view enables us to explain how there can be knowledge
a priori; and, in addition, to furnish satisfactory proofs of the
laws which form the a priori basis of nature, regarded as the
sum of the objects of experience -- neither achievement being
possible on the procedure hitherto followed. 
 This method, modelled on that of the student of nature, con-
sists in looking for the elements of pure reason in what admits of con-
firmation or refutation by experiment. Now the propositions of pure
reason, especially if they venture out beyond all limits of possible
experience, cannot be brought to the test through any experiment
with their objects, as in natural science. In dealing with those con-
cepts and principles which we adopt a priori, all that we can do is to
contrive that they be used for viewing objects from two different
points of view -- on the one hand, in connection with experience, as
objects of the senses and of the understanding, and on the other
hand, for the isolated reason that strives to transcend all limits of
experience, as objects which are thought merely. If, when things are
viewed from this twofold standpoint, we find that there is agreement
with the principle of pure reason, but that when we regard them
only from a single point of view reason is involved in unavoidable
self-conflict, the experiment decides in favour of the correctness of
this distinction. 
P 023
But this deduction
of our power of knowing a priori, in the first part of metaphysics,
has a consequence which is startling, and which has the appearance
P 024
of being highly prejudicial to the whole purpose of meta-
physics, as dealt with in the second part. For we are brought
to the conclusion that we can never transcend the limits of
possible experience, though that is precisely what this science
is concerned, above all else, to achieve. This situation yields,
however, just the very experiment by which, indirectly, we
are enabled to prove the truth of this first estimate of our
a priori knowledge of reason, namely, that such knowledge
has to do only with appearances, and must leave the thing
in itself as indeed real per se, but as not known by us. 
For what necessarily forces us to transcend the limits of
experience and of all appearances is the unconditioned,
which reason, by necessity and by right, demands in things
in themselves, as required to complete the series of con-
ditions. If, then, on the supposition that our empirical know-
ledge conforms to objects as things in themselves, we find
that the unconditioned cannot be thought without contradiction,
and that when, on the other hand, we suppose that our repre-
sentation of things, as they are given to us, does not conform
to these things as they are in themselves, but that these objects,
as appearances, conform to our mode of representation, the
contradiction vanishes; and if, therefore, we thus find that
the unconditioned is not to be met with in things, so far as
we know them, that is, so far as they are given to us, but
only so far as we do not know them, that is, so far as they
are things in themselves, we are justified in concluding that
what we at first assumed for the purposes of experiment is
now definitely confirmed. 
 This experiment of pure reason bears a great similarity to what
in chemistry is sometimes entitled the experiment of reduction, or
more usually the synthetic process. The analysis of the metaphysician
separates pure a priori knowledge into two very heterogeneous
elements, namely, the knowledge of things as appearances, and the
knowledge of things in themselves; his dialectic combines these
two again, in harmony with the necessary idea of the unconditioned
demanded by reason, and finds that this harmony can never be ob-
tained except through the above distinction, which must therefore
be accepted. 
P 024
But when all progress in the field
of the supersensible has thus been denied to speculative
reason, it is still open to us to enquire whether, in the practical
P 025
knowledge of reason, data may not be found sufficient to de-
termine reason's transcendent concept of the unconditioned,
and so to enable us, in accordance with the wish of meta-
physics, and by means of knowledge that is possible a priori,
though only from a practical point of view, to pass beyond
the limits of all possible experience. Speculative reason has
thus at least made room for such an extension; and if it must
at the same time leave it empty, yet none the less we are at
liberty, indeed we are summoned, to take occupation of it,
if we can, by practical data of reason. 
This attempt to alter the procedure which has hitherto
prevailed in metaphysics, by completely revolutionising it
in accordance with the example set by the geometers and
physicists, forms indeed the main purpose of this critique of
pure speculative reason. It is a treatise on the method, not a
system of the science itself. But at the same time it marks out
the whole plan of the science, both as regards its limits and as
regards its entire internal structure. For pure speculative reason
has this peculiarity, that it can measure its powers according
to the different ways in which it chooses the objects of its
thinking, and can also give an exhaustive enumeration of
the various ways in which it propounds its problems, and so
is able, nay bound, to trace the complete outline of a system
of metaphysics. As regards the first point, nothing in a priori
knowledge can be ascribed to objects save what the thinking
subject derives from itself; 
 Similarly, the fundamental laws of the motions of the heavenly
bodies gave established certainty to what Copernicus had at first
assumed only as an hypothesis, and at the same time yielded
proof of the invisible force (the Newtonian attraction) which holds
the universe together. The latter would have remained for ever un-
discovered if Copernicus had not dared, in a manner contradictory
of the senses, but yet true, to seek the observed movements, not in
the heavenly bodies, but in the spectator. The change in point of
view, analogous to this hypothesis, which is expounded in the
Critique, I put forward in this preface as an hypothesis only, in order
to draw attention to the character of these first attempts at such a
change, which are always hypothetical. But in the Critique itself it
will be proved, apodeictically not hypothetically, from the nature
of our representations of space and time and from the elementary
concepts of the understanding. 
P 025
 as regards the second point, pure
reason, so far as the principles of its knowledge are concerned,
P 026
is a quite separate self-subsistent unity, in which, as in an
organised body, every member exists for every other, and
all for the sake of each, so that no principle can safely be
taken in any one relation, unless it has been investigated in
the entirety of its relations to the whole employment of pure
reason. Consequently, metaphysics has also this singular
advantage, such as falls to the lot of no other science which
deals with objects (for logic is concerned only with the form
of thought in general), that should it, through this critique,
be set upon the secure path of a science, it is capable of ac-
quiring exhaustive knowledge of its entire field. Metaphysics
has to deal only with principles, and with the limits of their
employment as determined by these principles themselves,
and it can therefore finish its work and bequeath it to posterity
as a capital to which no addition can be made. Since it is
a fundamental science, it is under obligation to achieve this
completeness. We must be able to say of it: nil actum re-
putans, si quid superesset agendum. 
But, it will be asked, what sort of a treasure is this that
we propose to bequeath to posterity? What is the value of
the metaphysics that is alleged to be thus purified by criti-
cism and established once for all? On a cursory view of the
present work it may seem that its results are merely negative,
warning us that we must never venture with speculative reason
beyond the limits of experience. Such is in fact its primary use. 
But such teaching at once acquires a positive value when we
recognise that the principles with which speculative reason
ventures out beyond its proper limits do not in effect extend
the employment of reason, but, as we find on closer scrutiny,
inevitably narrow it. These principles properly belong [not
to reason but] to sensibility, and when thus employed they
threaten to make the bounds of sensibility coextensive with
the real, and so to supplant reason in its pure (practical) em-
ployment. So far, therefore, as our Critique limits speculative
reason, it is indeed negative; but since it thereby removes an
obstacle which stands in the way of the employment of practi-
cal reason, nay threatens to destroy it, it has in reality a posi-
tive and very important use. At least this is so, immediately
we are convinced that there is an absolutely necessary prac-
tical employment of pure reason -- the moral -- in which it
P 027
inevitably goes beyond the limits of sensibility. Though
[practical] reason, in thus proceeding, requires no assistance
from speculative reason, it must yet be assured against its
opposition, that reason may not be brought into conflict
with itself. To deny that the service which the Critique renders
is Positive in character, would thus be like saying that the
police are of no positive benefit, inasmuch as their main busi-
ness is merely to prevent the violence of which citizens stand
in mutual fear, in order that each may pursue his vocation in
peace and security. That space and time are only forms of sens-
ible intuition, and so only conditions of the existence of things
as appearances; that, moreover, we have no concepts of under-
standing, and consequently no elements for the knowledge of
things, save in so far as intuition can be given corresponding
to these concepts; and that we can therefore have no knowledge
of any object as thing in itself, but only in so far as it is an
object of sensible intuition, that is, an appearance -- all this is
proved in the analytical part of the Critique. Thus it does in-
deed follow that all possible speculative knowledge of reason
is limited to mere objects of experience. But our further con-
tention must also be duly borne in mind, namely, that though
We cannot know these objects as things in themselves, we
must yet be in position at least to think them as things in them-
selves; otherwise we should be landed in the absurd conclusion
that there can be appearance without anything that appears. 
Now let us suppose that the distinction, which our Critique has
shown to be necessary, between things as objects of experience
and those same things as things in themselves, had not been
 To know an object I must be able to prove its possibility, either
from its actuality as attested by experience, or a priori by means of
reason. But I can think whatever I please, provided only that I do
not contradict myself, that is, provided my concept is a possible
thought. This suffices for the possibility of the concept, even though
I may not be able to answer for there being, in the sum of all possi-
bilities, an object corresponding to it. But something more is re-
quired before I can ascribe to such a concept objective validity, that
is, real possibility; the former possibility is merely logical. This some-
thing more need not, however, be sought in the theoretical sources of
knowledge; it may lie in those that are practical. 
P 027
In that case all things in general, as far as they are
P 028
efficient causes, would be determined by the principle of caus-
ality and consequently by the mechanism of nature. I could
not, therefore, without palpable contradiction, say of one and
the same being, for instance the human soul, that its will is free
and yet is subject to natural necessity, that is, is not free. For
I have taken the soul in both propositions in one and the same
sense, namely as a thing in general, that is, as a thing in itself;
and save by means of a preceding critique, could not have done
otherwise. But if our Critique is not in error in teaching that
the object is to be taken in a twofold sense, namely as appear-
ance and as thing in itself; if the deduction of the concepts of
understanding is valid, and the principle of causality there-
fore applies only to things taken in the former sense, namely,
in so far as they are objects of experience -- these same objects,
taken in the other sense, not being subject to the principle --
then there is no contradiction in supposing that one and the
same will is, in the appearance, that is, in its visible acts,
necessarily subject to the law of nature, and so far not free,
while yet, as belonging to a thing in itself, it is not subject
to that law, and is therefore free. My soul, viewed from the
latter standpoint, cannot indeed be known by means of specu-
lative reason (and still less through empirical observation);
and freedom as a property of a being to which I attribute effects
in the sensible world, is therefore also not knowable in any
such fashion. For I should then have to know such a being as
determined in its existence, and yet as not determined in time --
which is impossible, since I cannot support my concept by any
intuition. But though I cannot know, I can yet think freedom;
that is to say, the representation of it is at least not self-con-
tradictory, provided due account be taken of our critical dis-
tinction between the two modes of representation, the sensible
and the intellectual, and of the resulting limitation of the pure
concepts of understanding and of the principles which flow
from them. 
If we grant that morality necessarily presupposes freedom
(in the strictest sense) as a property of our will; if, that is to
 say, we grant that it yields practical principles -- original prin-
 ciples, proper to our reason -- as a priori data of reason, and
that this would be absolutely impossible save on the assump-
P 029
tion of freedom; and if at the same time we grant that
speculative reason has proved that such freedom does not
allow of being thought, then the former supposition -- that
made on behalf of morality -- would have to give way to this
other contention, the opposite of which involves a palpable
contradiction. For since it is only on the assumption of free-
dom that the negation of morality contains any contradiction,
freedom, and with it morality, would have to yield to the
mechanism of nature. 
Morality does not, indeed, require that freedom should be
understood, but only that it should not contradict itself, and
so should at least allow of being thought, and that as thus
thought it should place no obstacle in the way of a free act
(viewed in another relation) likewise conforming to the mechan-
ism of nature. The doctrine of morality and the doctrine of
nature may each, therefore, make good its position. This,
however, is only possible in so far as criticism has previously
established our unavoidable ignorance of things in themselves,
and has limited all that we can theoretically know to mere
This discussion as to the positive advantage of critical
principles of pure reason can be similarly developed in regard
to the concept of God and of the simple nature of our soul; but
for the sake of brevity such further discussion may be omitted. 
[From what has already been said, it is evident that] even the
assumption--as made on behalf of the necessary practical em-
ployment of my reason -- of God, freedom, and immortality is
not permissible unless at the same time speculative reason be
deprived of its pretensions to transcendent insight. For in order
to arrive at such insight it must make use of principles which,
in fact, extend only to objects of possible experience, and
which, if also applied to what cannot be an object of experience,
always really change this into an appearance, thus rendering
all practical extension of pure reason impossible. I have therefore
found it necessary to deny knowledge, in order to make room
for faith. The dogmatism of metaphysics, that is, the precon-
ception that it is possible to make headway in metaphysics with-
out a previous criticism of pure reason, is the source of all that
unbelief, always very dogmatic, which wars against morality. 
P 030
Though it may not, then, be very difficult to leave to pos-
terity the bequest of a systematic metaphysic, constructed in
conformity with a critique of pure reason, yet such a gift is
not to be valued lightly. For not only will reason be enabled
to follow the secure path of a science, instead of, as hitherto,
groping at random, without circumspection or self-criticism;
our enquiring youth will also be in a position to spend
their time more profitably than in the ordinary dogmatism
by which they are so early and so greatly encouraged to
indulge in easy speculation about things of which they
understand nothing, and into which neither they nor any-
one else will ever have any insight -- encouraged, indeed, to
invent new ideas and opinions, while neglecting the study
of the better-established sciences. But, above all, there is
the inestimable benefit, that all objections to morality and
religion will be for ever silenced, and this in Socratic fashion,
namely, by the clearest proof of the ignorance of the objectors. 
There has always existed in the world, and there will always
continue to exist, some kind of metaphysics, and with it the
dialectic that is natural to pure reason. It is therefore the first
and most important task of philosophy to deprive meta-
physics, once and for all, of its injurious influence, by attack-
ing its errors at their very source. 
Notwithstanding this important change in the field of the
sciences, and the loss of its fancied possessions which specula-
tive reason must suffer, general human interests remain in the
same privileged position as hitherto, and the advantages which
the world has hitherto derived from the teachings of pure
reason are in no way diminished. The loss affects only the
monopoly of the schools, in no respect the interests of humanity. 
I appeal to the most rigid dogmatist, whether the proof of the
continued existence of our soul after death, derived from the
simplicity of substance, or of the freedom of the will as opposed
to a universal mechanism, arrived at through the subtle but
ineffectual distinctions between subjective and objective prac-
tical necessity, or of the existence of God as deduced from the
concept of an ens realissimum (of the contingency of the
changeable and of the necessity of a prime mover), have ever,
upon passing out from the schools, succeeded in reaching the
public mind or in exercising the slightest influence on its con-
P 031
victions? That has never been found to occur, and in view of
the unfitness of the common human understanding for such
subtle speculation, ought never to have been expected. Such
widely held convictions, so far as they rest on rational grounds,
are due to quite other considerations. The hope of a future life
has its source in that notable characteristic of our nature,
never to be capable of being satisfied by what is temporal (as
insufficient for the capacities of its whole destination); the
consciousness of freedom rests exclusively on the clear ex-
hibition of duties, in opposition to all claims of the inclina-
tions; the belief in a wise and great Author of the world is
generated solely by the glorious order, beauty, and providen-
tial care everywhere displayed in nature. When the schools
have been brought to recognise that they can lay no claim
to higher and fuller insight in a matter of universal human
concern than that which is equally within the reach of the
great mass of men (ever to be held by us in the highest
esteem), and that, as Schools of philosophy, they should limit
themselves to the study of those universally comprehensible,
and, for moral purposes, sufficient grounds of proof, then
not only do these latter possessions remain undisturbed, but
through this very fact they acquire yet greater authority. The
change affects only the arrogant pretensions of the Schools,
which would fain be counted the sole authors and possessors
of such truths (as, indeed, they can justly claim to be in many
other branches of knowledge), reserving the key to themselves,
and communicating to the public their use only -- quod mecum
nescit, solus vult scire videri. At the same time due regard is
paid to the more moderate claims of the speculative philosopher. 
He still remains the sole authority in regard to a science which
benefits the public without their knowing it, namely, the critique
of reason. That critique can never become popular, and indeed
there is no need that it should. For just as fine-spun arguments
in favour of useful truths make no appeal to the general mind,
so neither do the subtle objections that can be raised against
them. On the other hand, both inevitably present themselves
to everyone who rises to the height of speculation; and it is
therefore the duty of the Schools, by means of a thorough
investigation of the rights of speculative reason, once for all
to prevent the scandal which, sooner or later, is sure to
P 032
break out even among the masses, as the result of the
disputes in which metaphysicians (and, as such, finally also
the clergy) inevitably become involved to the consequent
perversion of their teaching. Criticism alone can sever the
root of materialism, fatalism, atheism, free-thinking, fanati-
cism, and superstition, which can be injurious universally; as
well as of idealism and scepticism, which are dangerous chiefly
to the Schools, and hardly allow of being handed on to the
public. If governments think proper to interfere with the
affairs of the learned, it would be more consistent with a wise
regard for science as well as for mankind, to favour the free-
dom of such criticism, by which alone the labours of reason
can be established on a firm basis, than to support the
ridiculous despotism of the Schools, which raise a loud cry of
public danger over the destruction of cobwebs to which the
public has never paid any attention, and the loss of which it
can therefore never feel. 
This critique is not opposed to the dogmatic procedure of
reason in its pure knowledge, as science, for that must always
be dogmatic, that is, yield strict proof from sure principles
a priori. It is opposed only to dogmatism, that is, to the pre-
sumption that it is possible to make progress with pure know-
ledge, according to principles, from concepts alone (those that
are philosophical), as reason has long been in the habit of
doing; and that it is possible to do this without having first in-
vestigated in what way and by what right reason has come into
possession of these concepts. Dogmatism is thus the dogmatic
procedure of pure reason, without previous criticism of its own
powers. In withstanding dogmatism we must not allow ourselves
to give free rein to that loquacious shallowness, which assumes
for itself the name of popularity, nor yet to scepticism, which
makes short work with all metaphysics. On the contrary, such
criticism is the necessary preparation for a thoroughly grounded
metaphysics, which, as science, must necessarily be developed
dogmatically, according to the strictest demands of system,
in such manner as to satisfy not the general public but the re-
quirements of the Schools. For that is a demand to which it
stands pledged, and which it may not neglect, namely, that it
carry out its work entirely a priori, to the complete satisfaction
of speculative reason. In the execution of the plan prescribed
P 033
by the critique, that is, in the future system of metaphysics
we have therefore to follow the strict method of the celebrated
Wolff, the greatest of all the dogmatic philosophers. He was
the first to show by example (and by his example he awakened
that spirit of thoroughness which is not extinct in Germany)
how the secure progress of a science is to be attained only
through orderly establishment of principles, clear determina-
tion of concepts, insistence upon strictness of proof, and avoid-
ance of venturesome, non-consecutive steps in our inferences. 
He was thus peculiarly well fitted to raise metaphysics to the
dignity of a science, if only it had occurred to him to prepare
the ground beforehand by a critique of the organ, that is, of
pure reason itself. The blame for his having failed to do so
lies not so much with himself as with the dogmatic way
of thinking prevalent in his day, and with which the philo-
sophers of his time, and of all previous times, have no right
to reproach one another. Those who reject both the method
of Wolff and the procedure of a critique of pure reason can
have no other aim than to shake off the fetters of science
altogether, and thus to change work into play, certainty into
opinion, philosophy into philodoxy. 
Now, as regards this second edition, I have, as is fitting,
endeavoured to profit by the opportunity, in order to remove,
wherever possible, difficulties and obscurity which, not per-
haps without my fault, may have given rise to the many
misunderstandings into which even acute thinkers have fallen
in passing judgment upon my book. In the propositions them-
selves and their proofs, and also in the form and completeness
of the [architectonic] plan, I have found nothing to alter. This
is due partly to the long examination to which I have sub-
jected them, before offering them to the public, partly to the
nature of the subject-matter with which we are dealing. For
pure speculative reason has a structure wherein everything
is an organ, the whole being for the sake of every part, and
every part for the sake of all the others, so that even the
smallest imperfection, be it a fault (error) or a deficiency, must
inevitably betray itself in use. This system will, as I hope,
maintain, throughout the future, this unchangeableness. It
is not self-conceit which justifies me in this confidence, but
P 034
the evidence experimentally obtained through the parity of
the result, whether we proceed from the smallest elements
to the whole of pure reason or reverse-wise from the whole
(for this also is presented to reason through its final end
in the sphere of the practical) to each part. Any attempt to
change even the smallest part at once gives rise to contradic-
tions, not merely in the system, but in human reason in
general. As to the mode of exposition, on the other hand,
much still remains to be done; and in this edition I have
sought to make improvements which should help in removing,
first, the misunderstanding in regard to the Aesthetic, especi-
ally concerning the concept of time; secondly, the obscurity
of the deduction of the concepts of understanding; thirdly, a
supposed want of sufficient evidence in the proofs of the prin-
ciples of pure understanding; and finally, the false interpreta-
tion placed upon the paralogisms charged against rational
psychology. Beyond this point, that is, beyond the end of the
first chapter of the Transcendental Dialectic, I have made no
changes in the mode of exposition. Time was too short to
P 035
allow of further changes; 
P 034n
 The only addition, strictly so called, though one affecting the
method of proof only, is the new refutation of psychological idealism
(cf. below, p. 244), and a strict (also, as I believe, the only possible)
proof of the objective reality of outer intuition. However harmless
idealism may be considered in respect of the essential aims of meta-
physics (though, in fact, it is not thus harmless), it still remains a
scandal to philosophy and to human reason in general that the
existence of things outside us (from which we derive the whole
material of knowledge, even for our inner sense) must be accepted
merely on faith, and that if anyone thinks good to doubt their exist-
ence, we are unable to counter his doubts by any satisfactory proof. 
Since there is some obscurity in the expressions used in the proof,
from the third line to the sixth line, I beg to alter the passage as
follows: "But this permanent cannot be an intuition in me. For all
grounds of determination of my existence which are to be met with
in me are representations; and as representations themselves require a
permanent distinct from them, in relation to which their change,
and so my existence in the time wherein they change, may be deter-
mined. To this proof it will probably be objected, that I am im-
mediately conscious only of that which is in me, that is, of my repre-
sentation of outer things; and consequently that it must still remain
uncertain whether outside me there is anything corresponding to it,
or not. 
P 035
 and besides, I have not found among
competent and impartial critics any misapprehension in regard
to the remaining sections. Though I shall not venture to name
these critics with the praise that is their due, the attention
which I have paid to their comments will easily be recognised
in the [new] passages [above mentioned]. These improvements
involve, however, a small loss, not to be prevented save by
making the book too voluminous, namely, that I have had
to omit or abridge certain passages, which, though not
indeed essential to the completeness of the whole, may yet
be missed by many readers as otherwise helpful. Only so
could I obtain space for what, as I hope, is now a more
intelligible exposition, which, though altering absolutely
nothing in the fundamentals of the propositions put for-
ward or even in their proofs, yet here and there departs
so far from the previous method of treatment, that mere in-
terpolations could not be made to suffice. This loss, which is
small and can be remedied by consulting the first edition, will,
I hope, be compensated by the greater clearness of the new
P 036
P 034n
But through inner experience I am conscious of my existence
P 035n
in time (consequently also of its determinability in time), and this is
more than to be conscious merely of my representation. It is identical
with the empirical consciousness of my existence, which is determin-
able only through relation to something which, while bound up with
my existence, is outside me. This consciousness of my existence in
time is bound up in the way of identity with the consciousness of a
relation to something outside me, and it is therefore experience not
invention, sense not imagination, which inseparably connects this
outside something with my inner sense. For outer sense is already
in itself a relation of intuition to something actual outside me, and
the reality of outer sense, in its distinction from imagination, rests
simply on that which is here found to take place, namely, its being
inseparably bound up with inner experience, as the condition of its
possibility. If, with the intellectual consciousness of my existence, in
the representation 'I am', which accompanies all my judgments and
acts of understanding, I could at the same time connect a determina-
tion of my existence through intellectual intuition, the conscious-
ness of a relation to something outside me would not be required. 
But though that intellectual consciousness does indeed come first,
the inner intuition, in which my existence can alone be determined,
is sensible and is bound up with the condition of time. This deter-
mination, however, and therefore the inner experience itself, depends
P 036n
upon something permanent which is not in me, and consequently
can be only in something outside me, to which I must regard my-
self as standing in relation. 
P 036
I have observed, with pleasure and thankfulness, in
various published works -- alike in critical reviews and in in-
dependent treatises -- that the spirit of thoroughness is not
extinct in Germany, but has only been temporarily over-
shadowed by the prevalence of a pretentiously free manner of
thinking; and that the thorny paths of the Critique have not
discouraged courageous and clear heads from setting them-
selves to master my book -- a work which leads to a method-
ical, and as such alone enduring, and therefore most necessary,
science of pure reason. To these worthy men, who so happily
combine thoroughness of insight with a talent for lucid ex-
position -- which I cannot regard myself as possessing -- I
leave the task of perfecting what, here and there, in its
exposition, is still somewhat defective; for in this regard
the danger is not that of being refuted, but of not being
P 037
P 036n
The reality of outer sense is thus neces-
sarily bound up with inner sense, if experience in general is to be
possible at all; that is, I am just as certainly conscious that there are
things outside me, which are in relation to my sense, as I am con-
scious that I myself exist as determined in time. In order to deter-
mine to which given intuitions objects outside me actually corre-
spond, and which therefore belong to outer sense (to which, and not
to the faculty of imagination, they are to be ascribed), we must in
each single case appeal to the rules according to which experience
in general, even inner experience, is distinguished from imagination
 -- the proposition that there is such a thing as outer experience being
always presupposed. This further remark may be added. The repre-
sentation of something permanent in existence is not the same as
permanent representation. For though the representation of [some-
thing permanent] may be very transitory and variable like all our
other representations, not excepting those of matter, it yet refers to
something permanent. This latter must therefore be an external
thing distinct from all my representations, and its existence must be
included in the determination of my own existence, constituting with
it but a single experience such as would not take place even inwardly
if it were not also at the same time, in part, outer. How this should
be possible we are as little capable of explaining further as we are of
accounting for our being able to think the abiding in time, the co-
existence of which with the changing generates the concept of altera-
P 037
From now on, though I cannot allow myself to
enter into controversy, I shall take careful note of all sugges-
tions, be they from friends or from opponents, for use, in
accordance with this propaedeutic, in the further elaboration
of the system. In the course of these labours I have advanced
somewhat far in years (this month I reach my sixty-fourth
year), and I must be careful with my time if I am to succeed
in my proposed scheme of providing a metaphysic of nature
and of morals which will confirm the truth of my Critique in
the two fields, of speculative and of practical reason. The
clearing up of the obscurities in the present work -- they are
hardly to be avoided in a new enterprise -- and the defence
of it as a whole, I must therefore leave to those worthy men
who have made my teaching their own. A philosophical work
cannot be armed at all points, like a mathematical treatise,
and may therefore be open to objection in this or that respect,
while yet the structure of the system, taken in its unity, is not
in the least endangered. Few have the versatility of mind to
familiarise themselves with a new system; and owing to the
general distaste for all innovation, still fewer have the inclina-
tion to do so. If we take single passages, torn from their
contexts, and compare them with one another, apparent con-
tradictions are not likely to be lacking, especially in a work
that is written with any freedom of expression. In the eyes of
those who rely on the judgment of others, such contradic-
tions have the effect of placing the work in an unfavourable
light; but they are easily resolved by those who have mastered
the idea of the whole. If a theory has in itself stability, the
stresses and strains which may at first have seemed very
threatening to it serve only, in the course of time, to smooth
away its inequalities; and if men of impartiality, insight, and
true popularity devote themselves to its exposition, it may also,
in a short time, secure for itself the necessary elegance of
Konigsberg, April 1787. 
P 041
THERE can be no doubt that all our knowledge begins with
experience. For how should our faculty of knowledge be
awakened into action did not objects affecting our senses
partly of themselves produce representations, partly arouse
the activity of our understanding to compare these repre-
sentations, and, by combining or separating them, work
up the raw material of the sensible impressions into that
knowledge of objects which is entitled experience? In the
order of time, therefore, we have no knowledge antecedent to
experience, and with experience all our knowledge begins. 
But though all our knowledge begins with experience,
it does not follow that it all arises out of experience. 
Experience is, beyond all doubt, the first product to which
our understanding gives rise, in working up the raw material
of sensible impressions. Experience is therefore our first
instruction, and in its progress is so inexhaustible in new
information, that in the interconnected lives of all future
generations there will never be any lack of new knowledge
that can be thus ingathered. Nevertheless, it is by no means
P 042a
the sole field to which our understanding is confined. 
P 041
For it
P 042
may well be that even our empirical knowledge is made up of
what we receive through impressions and of what our own
faculty of knowledge (sensible impressions serving merely as
the occasion) supplies from itself. If our faculty of knowledge
makes any such addition, it may be that we are not in a posi-
tion to distinguish it from the raw material, until with long
practice of attention we have become skilled in separating it. 
This, then, is a question which at least calls for closer
examination, and does not allow of any off-hand answer: --
whether there is any knowledge that is thus independent of
experience and even of all impressions of the senses. Such
knowledge is entitled a priori, and distinguished from the
P 043
empirical, which has its sources a posteriori, that is, in experi-
P 042a
ence tells us, indeed, what is, but not that it must necessarily
be so, and not otherwise. It therefore gives us no true
universality; and reason, which is so insistent upon this
kind of knowledge, is therefore more stimulated by it than
satisfied. Such universal modes of knowledge, which at the
same time possess the character of inner necessity, must in
themselves, independently of experience, be clear and certain. 
They are therefore entitled knowledge a priori; whereas, on
the other hand, that which is borrowed solely from experience
is, as we say, known only a posteriori, or empirically. 
Now we find, what is especially noteworthy, that even into
our experiences there enter modes of knowledge which must
have their origin a priori, and which perhaps serve only to
give coherence to our sense-representations. For if we elimin-
ate from our experiences everything which belongs to the
senses, there still remain certain original concepts and certain
judgments derived from them, which must have arisen com-
pletely a priori, independently of experience, inasmuch as
they enable us to say, or at least lead us to believe that we can
say, in regard to the objects which appear to the senses, more
than mere experience would teach -- giving to assertions true
universality and strict necessity, such as mere empirical know-
ledge cannot supply. 
P 043
The expression 'a priori' does not, however, indicate with
sufficient precision the full meaning of our question. For it
has been customary to say, even of much knowledge that is
derived from empirical sources, that we have it or are capable
of having it a priori, meaning thereby that we do not derive
it immediately from experience, but from a universal rule -- a
rule which is itself, however, borrowed by us from experience. 
Thus we would say of a man who undermined the foundations
of his house, that he might have known a priori that it would
fall, that is, that he need not have waited for the experience of
its actual falling. But still he could not know this completely
a priori. For he had first to learn through experience that
bodies are heavy, and therefore fall when their supports are
In what follows, therefore, we shall understand by a priori
knowledge, not knowledge independent of this or that experi-
ence, but knowledge absolutely independent of all experience. 
Opposed to it is empirical knowledge, which is knowledge
possible only a posteriori, that is, through experience. A -
priori modes of knowledge are entitled pure when there is
no admixture of anything empirical. Thus, for instance, the
proposition, 'every alteration has its cause', while an a priori
proposition, is not a pure proposition, because alteration is a
concept which can be derived only from experience. 
What we here require is a criterion by which to distinguish
with certainty between pure and empirical knowledge. Ex-
perience teaches us that a thing is so and so, but not that it
cannot be otherwise. First, then, if we have a proposition
which in being thought is thought as necessary, it is an a priori
judgment; and if, besides, it is not derived from any proposi-
tion except one which also has the validity of a necessary
judgment, it is an absolutely a priori judgment. Secondly,
P 044
experience never confers on its judgments true or strict but
only assumed and comparative universality, through induc-
tion. We can properly only say, therefore, that so far as
we have hitherto observed, there is no exception to this or
that rule. If, then, a judgment is thought with strict univer-
sality, that is, in such manner that no exception is allowed as
possible, it is not derived from experience, but is valid abso-
lutely a priori. Empirical universality is only an arbitrary ex-
tension of a validity holding in most cases to one which holds
in all, for instance, in the proposition, 'all bodies are heavy'. 
When, on the other hand, strict universality is essential to a
a judgment, this indicates a special source of knowledge,
namely, a faculty of a priori knowledge. Necessity and strict
universality are thus sure criteria of a priori knowledge, and
are inseparable from one another. But since in the employ-
ment of these criteria the contingency of judgments is some-
times more easily shown than their empirical limitation, or,
as sometimes also happens, their unlimited universality can
be more convincingly proved than their necessity, it is advis-
able to use the two criteria separately, each by itself being
Now it is easy to show that there actually are in human
knowledge judgments which are necessary and in the strictest
sense universal, and which are therefore pure a priori judg-
ments. If an example from the sciences be desired, we have
only to look to any of the propositions of mathematics; if we
seek an example from the understanding in its quite ordinary
employment, the proposition, 'every alteration must have a
cause', will serve our purpose. In the latter case, indeed, the
very concept of a cause so manifestly contains the concept of
a necessity of connection with an effect and of the strict uni-
versality of the rule, that the concept would be altogether lost if
we attempted to derive it, as Hume has done, from a repeated
association of that which happens with that which precedes,
and from a custom of connecting representations, a custom
originating in this repeated association, and constituting
therefore a merely subjective necessity. Even without appeal-
P 045
ing to such examples, it is possible to show that pure a priori
principles are indispensable for the possibility of experience,
and so to prove their existence a priori. For whence could
experience derive its certainty, if all the rules, according to
which it proceeds, were always themselves empirical, and
therefore contingent? Such rules could hardly be regarded as
first principles. At present, however, we may be content to
have established the fact that our faculty of knowledge does
have a pure employment, and to have shown what are the
criteria of such an employment. 
Such a priori origin is manifest in certain concepts, no
less than in judgments. If we remove from our empirical
concept of a body, one by one, every feature in it which is
[merely] empirical, the colour, the hardness or softness, the
weight, even the impenetrability, there still remains the
space which the body (now entirely vanished) occupied, and
this cannot be removed. Again, if we remove from our em-
pirical concept of any object, corporeal or incorporeal, all
properties which experience has taught us, we yet cannot take
away that property through which the object is thought as
substance or as inhering in a substance (although this concept
of substance is more determinate than that of an object in
general). Owing, therefore, to the necessity with which this
concept of substance forces itself upon us, we have no option
save to admit that it has its seat in our faculty of a priori
But what is still more extraordinary than all the preceding
is this, that certain modes of knowledge leave the field of all
possible experiences and have the appearance of extending
the scope of our judgments beyond all limits of experience,
and this by means of concepts to which no corresponding
object can ever be given in experience. 
It is precisely by means of the latter modes of knowledge,
in a realm beyond the world of the senses, where experience
P 046
can yield neither guidance nor correction, that our reason
carries on those enquiries which owing to their importance
we consider to be far more excellent, and in their purpose
far more lofty, than all that the understanding can learn in
the field of appearances. Indeed we prefer to run every risk
of error rather than desist from such urgent enquiries, on the
ground of their dubious character, or from disdain and in-
difference. These unavoidable problems set by pure reason
itself are God, freedom, and immortality. The science which,
with all its preparations, is in its final intention directed
solely to their solution is metaphysics; and its procedure
is at first dogmatic, that is, it confidently sets itself to this
task without any previous examination of the capacity or
incapacity of reason for so great an undertaking. 
Now it does indeed seem natural that, as soon as we have
left the ground of experience, we should, through careful en-
quiries, assure ourselves as to the foundations of any building
that we propose to erect, not making use of any knowledge
that we possess without first determining whence it has come,
and not trusting to principles without knowing their origin. 
It is natural, that is to say, that the question should first be
considered, how the understanding can arrive at all this know-
ledge a priori, and what extent, validity, and worth it may
have. Nothing, indeed, could be more natural, if by the term
'natural' we signify what fittingly and reasonably ought to
happen. But if we mean by 'natural' what ordinarily happens,
then on the contrary nothing is more natural and more in-
telligible than the fact that this enquiry has been so long neg-
lected. For one part of this knowledge, the mathematical, has
long been of established reliability, and so gives rise to a favour-
able presumption as regards the other part, which may yet be of
quite different nature. Besides, once we are outside the circle
of experience, we can be sure of not being contradicted by
experience. The charm of extending our knowledge is so
great that nothing short of encountering a direct contra-
diction can suffice to arrest us in our course; and this can be
avoided, if we are careful in our fabrications -- which none the
less will still remain fabrications. Mathematics gives us a shin-
P 047
ing example of how far, independently of experience, we can
progress in a priori knowledge. It does, indeed, occupy itself
with objects and with knowledge solely in so far as they allow
of being exhibited in intuition. But this circumstance is easily
overlooked, since the intuition, in being thought, can itself be
given a priori, and is therefore hardly to be distinguished from
a bare and pure concept. Misled by such a proof of the power
of reason, the demand for the extension of knowledge recog-
nises no limits. The light dove, cleaving the air in her free
flight, and feeling its resistance, might imagine that its flight
would be still easier in empty space. It was thus that Plato
left the world of the senses, as setting too narrow limits to
the understanding, and ventured out beyond it on the wings
of the ideas, in the empty space of the pure understanding. 
He did not observe that with all his efforts he made no ad-
vance -- meeting no resistance that might, as it were, serve
as a support upon which he could take a stand, to which
he could apply his powers, and so set his understanding
in motion. It is, indeed, the common fate of human reason
to complete its speculative structures as speedily as may
be, and only afterwards to enquire whether the foundations
are reliable. All sorts of excuses will then be appealed to, in
order to reassure us of their solidity, or rather indeed to
enable us to dispense altogether with so late and so dangerous
an enquiry. But what keeps us, during the actual building,
free from all apprehension and suspicion, and flatters us with
a seeming thoroughness, is this other circumstance, namely,
that a great, perhaps the greatest, part of the business of our
reason consists in analysis of the concepts which we already
have of objects. This analysis supplies us with a consider-
able body of knowledge, which, while nothing but explanation
or elucidation of what has already been thought in our con-
cepts, though in a confused manner, is yet prized as being,
at least as regards its form, new insight. But so far as the
matter or content is concerned, there has been no extension of
our previously possessed concepts, but only an analysis of them. 
Since this procedure yields real knowledge a priori, which
P 048
progresses in an assured and useful fashion, reason is so far
misled as surreptitiously to introduce, without itself being
aware of so doing, assertions of an entirely different order, in
which it attaches to given concepts others completely foreign
to them, and moreover attaches them a priori. And yet it is
not known how reason can be in position to do this. Such a
question is never so much as thought of. I shall therefore
at once proceed to deal with the difference between these two
kinds of knowledge. 
In all judgments in which the relation of a subject to the
predicate is thought (I take into consideration affirmative
judgments only, the subsequent application to negative judg-
ments being easily made), this relation is possible in two
different ways. Either the predicate to the subject
A, as something which is (covertly) contained in this concept
A; or outside the concept A, although it does indeed
stand in connection with it. In the one case I entitle the judg-
ment analytic, in the other synthetic. Analytic judgments
(affirmative) are therefore those in which the connection of the
predicate with the subject is thought through identity; those
in which this connection is thought without identity should
 be entitled synthetic. The former, as adding nothing through
the predicate to the concept of the subject, but merely break-
ing it up into those constituent concepts that have all along
been thought in it, although confusedly, can also be entitled
explicative. The latter, on the other hand, add to the concept
of the subject a predicate which has not been in any wise thought
in it, and which no analysis could possibly extract from it; and
they may therefore be entitled ampliative. If I say, for instance,
'All bodies are extended', this is an analytic judgment. For I
do not require to go beyond the concept which I connect with
'body' in order to find extension as bound up with it. To
P 049
meet with this predicate, I have merely to analyse the concept,
that is, to become conscious to myself of the manifold which
I always think in that concept. The judgment is therefore
analytic. But when I say, 'All bodies are heavy', the predi-
cate is something quite different from anything that I think in
the mere concept of body in general; and the addition of such
a predicate therefore yields a synthetic judgment. 
* Judgments of experience, as such, are one and all synthetic. 
For it would be absurd to found an analytic judgment on ex-
perience. Since, in framing the judgment, I must not go out-
side my concept, there is no need to appeal to the testimony
of experience in its support. That a body is extended is a pro-
position that holds a priori and is not empirical. For, before
appealing to experience, I have already in the concept of body
all the conditions required for my judgment. I have only to ex-
tract from it, in accordance with the principle of contradiction,
the required predicate, and in so doing can at the same time
become conscious of the necessity of the judgment -- and that
is what experience could never have taught me. On the other
hand, though I do not include in the concept of a body in
general the predicate 'weight', none the less this concept indi-
cates an object of experience through one of its parts, and I
can add to that part other parts of this same experience, as in
this way belonging together with the concept. 
*Thus it is evident: 1. that through analytic judgments our
knowledge is not in any way extended, and that the concept
which I already have is merely set forth and made intelligible
to me; 2. that in synthetic judgments I must have besides the
concept of the subject something else (X), upon which the un-
derstanding may rely, if it is to know that a predicate, not
contained in this concept, nevertheless belongs to it. 
In the case of empirical judgments, judgments of experi-
ence, there is no difficulty whatsoever in meeting this demand. 
This X is the complete experience of the object which I think
through the concept A -- a concept which forms only one part
of this experience. 
P 049
From the start
P 050
I can apprehend the concept of body analytically through the
characters of extension, impenetrability, figure, etc. , all of
which are thought in the concept. Now, however, looking
back on the experience from which I have derived this con-
cept of body, and finding weight to be invariably connected
with the above characters, I attach it as a predicate to the
concept; and in doing so I attach it synthetically, and am
therefore extending my knowledge. The possibility of the syn-
thesis of the predicate 'weight' with the concept of 'body' thus
rests upon experience. While the one concept is not contained
in the other, they yet belong to one another, though only con-
tingently, as parts of a whole, namely, of an experience which
is itself a synthetic combination of intuitions. 
 But in a priori synthetic judgments this help is entirely
lacking. [I do not here have the advantage of looking around
in the field of experience. ] Upon what, then, am I to rely, when
I seek to go beyond the concept A, and to know that another
concept B is connected with it? Through what is the syn-
thesis made possible? Let us take the proposition, 'Every-
thing which happens has its cause'. In the concept of 'some-
thing which happens', I do indeed think an existence which is
preceded by a time, etc. , and from this concept analytic judg-
ments may be obtained. 
P 049a
For though I do not include in the concept
P 050a
of a body in general the predicate 'weight', the concept none
the less indicates the complete experience through one of its
parts; and to this part, as belonging to it, I can therefore add
other parts of the same experience. By prior analysis I can ap-
prehend the concept of body through the characters of exten-
sion, impenetrability, figure, etc. , all of which are thought in
this concept. To extend my knowledge, I then look back to the
experience from which I have derived this concept of body, and
find that weight is always connected with the above characters. 
Experience is thus the X which lies outside the concept A,
and on which rests the possibility of the synthesis of the
predicate 'weight' (B) with the concept (A). 
P 050
But the concept of a 'cause' lies entirely
outside the other concept, and signifies something different
P 051
from 'that which happens', and is not therefore in any way
contained in this latter representation. How come I then to
predicate of that which happens something quite different,
and to apprehend that the concept of cause, though not con-
tained in it, yet belongs, and indeed necessarily belongs to it? 
What is here the unknown = X which gives support to the
understanding when it believes that it can discover outside
the concept A a predicate B foreign to this concept, which
it yet at the same time considers to be connected with it? 
It cannot be experience, because the suggested principle has
connected the second representation with the first, not only
with greater universality, but also with the character of
necessity, and therefore completely a priori and on the basis
of mere concepts. Upon such synthetic, that is, ampliative
principles, all our a priori speculative knowledge must ulti-
mately rest; analytic judgments are very important, and indeed
necessary, but only for obtaining that clearness in the con-
cepts which is requisite for such a sure and wide synthesis as
will lead to a genuinely new addition to all previous know-
* A certain mystery lies here concealed; and only upon
its solution can the advance into the limitless field of the
knowledge yielded by pure understanding be made sure and
trustworthy. What we must do is to discover, in all its proper
universality, the ground of the possibility of a priori synthetic
judgments, to obtain insight into the conditions which make
P 052a
each kind of such judgments possible, and to mark out all this
knowledge, which forms a genus by itself, not in any cursory
outline, but in a system, with completeness and in a manner
sufficient for any use, according to its original sources, divi-
sions, extent, and limits. So much, meantime, as regards
what is peculiar in synthetic judgments. 
P 051n
* If it had occurred to any of the ancients even to raise this
question, this by itself would, up to our own time, have been a power-
ful influence against all systems of pure reason, and would have
saved us so many of those vain attempts, which have been blindly
undertaken without knowledge of what it is that requires to be done. 
P 052
1. All mathematical judgments, without exception, are
synthetic. This fact, though incontestably certain and in
its consequences very important, has hitherto escaped the
notice of those who are engaged in the analysis of human
reason, and is, indeed, directly opposed to all their conjectures. 
For as it was found that all mathematical inferences proceed
in accordance with the principle of contradiction (which the
nature of all apodeictic certainty requires), it was supposed that
the fundamental propositions of the science can themselves be
known to be true through that principle. This is an erroneous
view. For though a synthetic proposition can indeed be dis-
cerned in accordance with the principle of contradiction, this
can only be if another synthetic proposition is presupposed,
and if it can then be apprehended as following from this other
proposition; it can never be so discerned in and by itself. 
First of all, it has to be noted that mathematical proposi-
tions, strictly so called, are always judgments a priori, not
empirical; because they carry with them necessity, which
 cannot be derived from experience. If this be demurred to,
I am willing to limit my statement to pure mathematics, the
very concept of which implies that it does not contain empirical,
but only pure a priori knowledge. 
We might, indeed, at first suppose that the proposition
7 & 5 = 12 is a merely analytic proposition, and follows by
the principle of contradiction from the concept of a sum of
7 and 5. But if we look more closely we find that the concept
of the sum of 7 and 5 contains nothing save the union of the
two numbers into one, and in this no thought is being taken
P 053
as to what that single number may be which combines both. 
The concept of 12 is by no means already thought in merely
thinking this union of 7 and 5; and I may analyse my concept
of such a possible sum as long as I please, still I shall never
find the 12 in it. We have to go outside these concepts, and
call in the aid of the intuition which corresponds to one of
them, our five fingers, for instance, or, as Segner does in his
Arithmetic, five points, adding to the concept of 7, unit by
unit, the five given in intuition. For starting with the number
7, and for the concept of 5 calling in the aid of the fingers of
my hand as intuition, I now add one by one to the number 7
the units which I previously took together to form the number
5, and with the aid of that figure [the hand] see the number 12
come into being. That 5 should be added to 7, I have indeed
already thought in the concept of a sum = 7 & 5, but not that
this sum is equivalent to the number 12. Arithmetical pro-
positions are therefore always synthetic. This is still more
evident if we take larger numbers. For it is then obvious that,
however we might turn and twist our concepts, we could
never, by the mere analysis of them, and without the aid of
intuition, discover what [the number is that] is the sum. 
Just as little is any fundamental proposition of pure
geometry analytic. That the straight line between two points
is the shortest, is a synthetic proposition. For my concept of
straight contains nothing of quantity, but only of quality. The
concept of the shortest is wholly an addition, and cannot be
derived, through any process of analysis, from the concept of
the straight line. Intuition, therefore, must here be called in;
only by its aid is the synthesis possible. What here causes
us commonly to believe that the predicate of such apodeictic
judgments is already contained in our concept, and that the
judgment is therefore analytic, is merely the ambiguous
character of the terms used. We are required to join in
thought a certain predicate to a given concept, and this neces-
P 054
sity is inherent in the concepts themselves. But the question is
not what we ought to join in thought to the given concept, but
what we actually think in it, even if only obscurely; and it is
then manifest that, while the predicate is indeed attached
necessarily to the concept, it is so in virtue of an intuition
which must be added to the concept, not as thought in the
concept itself. 
 Some few fundamental propositions, presupposed by the
geometrician, are, indeed, really analytic, and rest on the
principle of contradiction. But, as identical propositions, they
serve only as links in the chain of method and not as prin-
ciples; for instance, a = a; the whole is equal to itself; or
(a & b)  a, that is, the whole is greater than its part. And even
these propositions, though they are valid according to pure
concepts, are only admitted in mathematics because they can
be exhibited in intuition. 
2. Natural science (physics) contains a priori synthetic
judgments as principles. I need cite only two such judgments:
that in all changes of the material world the quantity of matter
remains unchanged; and that in all communication of motion,
action and reaction must always be equal. Both propositions,
it is evident, are not only necessary, and therefore in their origin
a priori, but also synthetic. For in the concept of matter I do
not think its permanence, but only its presence in the space
which it occupies. I go outside and beyond the concept of
matter, joining to it a priori in thought something which I
have not thought in it. The proposition is not, therefore, ana-
lytic, but synthetic, and yet is thought a priori; and so likewise
are the other propositions of the pure part of natural science. 
3. Metaphysics, even if we look upon it as having hitherto
failed in all its endeavours, is yet, owing to the nature of
human reason, a quite indispensable science, and ought to
contain a priori synthetic knowledge. For its business is not
merely to analyse concepts which we make for ourselves a -
priori of things, and thereby to clarify them analytically, but
to extend our a priori knowledge. And for this purpose we
must employ principles which add to the given concept some-
thing that was not contained in it, and through a priori syn-
thetic judgments venture out so far that experience is quite
P 055
unable to follow us, as, for instance, in the proposition, that
the world must have a first beginning, and such like. Thus
metaphysics consists, at least in intention, entirely of a priori
synthetic propositions. 
Much is already gained if we can bring a number of in-
vestigations under the formula of a single problem. For we
not only lighten our own task, by defining it accurately, but
make it easier for others, who would test our results, to judge
whether or not we have succeeded in what we set out to do. 
Now the proper problem of pure reason is contained in the
question: How are a priori synthetic judgments possible? 
That metaphysics has hitherto remained in so vacillating
a state of uncertainty and contradiction, is entirely due to the
fact that this problem, and perhaps even the distinction be-
tween analytic and synthetic judgments, has never previously
been considered. Upon the solution of this problem, or upon
a sufficient proof that the possibility which it desires to have
explained does in fact not exist at all, depends the success or
failure of metaphysics. Among philosophers, David Hume
came nearest to envisaging this problem, but still was very far
from conceiving it with sufficient definiteness and universality. 
He occupied himself exclusively with the synthetic proposi-
tion regarding the connection of an effect with its cause
(principium causalitatis), and he believed himself to have
shown that such an a priori proposition is entirely impos-
sible. If we accept his conclusions, then all that we call
metaphysics is a mere delusion whereby we fancy ourselves to
have rational insight into what, in actual fact, is borrowed
solely from experience, and under the influence of custom has
taken the illusory semblance of necessity. If he had envisaged
our problem in all its universality, he would never have been
guilty of this statement, so destructive of all pure philosophy. 
For he would then have recognised that, according to his own
argument, pure mathematics, as certainly containing a priori
synthetic propositions, would also not be possible; and from
such an assertion his good sense would have saved him. 
In the solution of the above problem, we are at the same
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time deciding as to the possibility of the employment of pure
reason in establishing and developing all those sciences which
contain a theoretical a priori knowledge of objects, and have
therefore to answer the questions:
How is pure mathematics possible? 
How is pure science of nature possible? 
Since these sciences actually exist, it is quite proper to ask
how they are possible; for that they must be possible is proved
by the fact that they exist. But the poor progress which has
hitherto been made in metaphysics, and the fact that no
system yet propounded can, in view of the essential purpose
of metaphysics, be said really to exist, leaves everyone suffi-
cient ground for doubting as to its possibility. 
Yet, in a certain sense, this kind of knowledge is to be
looked upon as given; that is to say, metaphysics actually
exists, if not as a science, yet still as natural disposition (meta-
physica naturalis). For human reason, without being moved
merely by the idle desire for extent and variety of knowledge,
proceeds impetuously, driven on by an inward need, to ques-
tions such as cannot be answered by any empirical employ-
ment of reason, or by principles thence derived. Thus in all
men, as soon as their reason has become ripe for speculation,
there has always existed and will always continue to exist
some kind of metaphysics. And so we have the question:
How is metaphysics, as natural disposition, possible? 
that is, how from the nature of universal human reason do
those questions arise which pure reason propounds to itself,
and which it is impelled by its own need to answer as best it
* Many may still have doubts as regards pure natural science. 
We have only, however, to consider the various propositions that are
to be found at the beginning of (empirical) physics, properly so
called, those, for instance, relating to the permanence in the quantity
of matter, to inertia, to the equality of action and reaction, etc. , in
order to be soon convinced that they constitute a physica pura, or
rationalis, which well deserves, as an independent science, to be
separately dealt with in its whole extent, be that narrow or wide. 
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But since all attempts which have hitherto been made
to answer these natural questions -- for instance, whether the
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world has a beginning or is from eternity -- have always met
with unavoidable contradictions, we cannot rest satisfied with
the mere natural disposition to metaphysics, that is, with the
pure faculty of reason itself, from which, indeed, some sort of
metaphysics (be it what it may) always arises. It must be
possible for reason to attain to certainty whether we know or
do not know the objects of metaphysics, that is, to come to
a decision either in regard to the objects of its enquiries or in
regard to the capacity or incapacity of reason to pass any
judgment upon them, so that we may either with confidence
extend our pure reason or set to it sure and determinate
limits. This last question, which arises out of the previous
general problem, may, rightly stated, take the form:
How is metaphysics, as science, possible? 
Thus the critique of reason, in the end, necessarily leads to
scientific knowledge; while its dogmatic employment, on the
other hand, lands us in dogmatic assertions to which other
assertions, equally specious, can always be opposed -- that is,
in scepticism. 
This science cannot be of any very formidable prolixity,
since it has to deal not with the objects of reason, the variety
of which is inexhaustible, but only with itself and the prob-
lems which arise entirely from within itself, and which are
imposed upon it by its own nature, not by the nature of things
which are distinct from it. When once reason has learnt com-
pletely to understand its own power in respect of objects which
can be presented to it in experience, it should easily be able to
determine, with completeness and certainty, the extent and
the limits of its attempted employment beyond the bounds of
all experience. 
We may, then, and indeed we must, regard as abortive all
attempts, hitherto made, to establish a metaphysic dogmatic-
ally. For the analytic part in any such attempted system,
namely, the mere analysis of the concepts that inhere in our
reason a priori, is by no means the aim of, but only a prepara-
tion for, metaphysics proper, that is, the extension of its a -
priori synthetic knowledge. For such a purpose, the analysis
of concepts is useless, since it merely shows what is contained
in these concepts, not how we arrive at them a priori. A solution
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of this latter problem is required, that we may be able to de-
termine the valid employment of such concepts in regard to
the objects of all knowledge in general. Nor is much self-denial
needed to give up these claims, seeing that the undeniable,
and in the dogmatic procedure of reason also unavoidable,
contradictions of reason with itself have long since undermined
the authority of every metaphysical system yet propounded. 
Greater firmness will be required if we are not to be deterred
by inward difficulties and outward opposition from endeavour-
ing, through application of a method entirely different from
any hitherto employed, at last to bring to a prosperous and
fruitful growth a science indispensable to human reason -- a
science whose every branch may be cut away but whose root
cannot be destroyed. 
In view of all these considerations, we arrive at the idea of
a special science which can be entitled the Critique of Pure
Reason. * For reason is the faculty which supplies the principles
of a priori knowledge. Pure reason is, therefore, that which
contains the principles whereby we know anything absolutely
a priori. An organon of pure reason would be the sum-total of
those principles according to which all modes of pure a priori
knowledge can be acquired and actually brought into being. 
The exhaustive application of such an organon would give
rise to a system of pure reason. But as this would be asking
rather much, and as it is still doubtful whether, and in what
cases, any extension of our knowledge be here possible, we
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can regard a science of the mere examination of pure reason,
of its sources and limits, as the propaedeutic to the system of
pure reason. 
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*Any knowledge is entitled pure, if it be not mixed with any-
thing extraneous. But knowledge is more particularly to be
called absolutely pure, if no experience or sensation whatso-
ever be mingled with it, and if it be therefore possible com-
pletely a priori. 
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As such, it should be called a critique, not a
doctrine, of pure reason. Its utility, in speculation, ought
properly to be only negative, not to extend, but only to clarify
our reason, and keep it free from errors -- which is already
a very great gain. I entitle transcendental all knowledge
which is occupied not so much with objects as with the
mode of our knowledge of objects in so far as this mode of
knowledge is to be possible a priori. A system of such con-
cepts might be entitled transcendental philosophy. But that
is still, at this stage, too large an undertaking. For since
such a science must contain, with completeness, both kinds
of a priori knowledge, the analytic no less than the synthetic,
it is, so far as our present purpose is concerned, much too
comprehensive. We have to carry the analysis so far only
as is indispensably necessary in order to comprehend, in
their whole extent, the principles of a priori synthesis, with
which alone we are called upon to deal. It is upon this
enquiry, which should be entitled not a doctrine, but only
a transcendental critique, that we are now engaged. Its pur-
pose is not to extend knowledge, but only to correct it, and to
supply a touchstone of the value, or lack of value, of all a priori
knowledge. Such a critique is therefore a preparation, so far
as may be possible, for an organon; and should this turn out
not to be possible, then at least for a canon, according to which,
in due course, the complete system of the philosophy of pure
reason -- be it in extension or merely in limitation of its know-
ledge -- may be carried into execution, analytically as well as
synthetically. That such a system is possible, and indeed that
it may not be of such great extent as to cut us off from the hope
of entirely completing it, may already be gathered from the
fact that what here constitutes our subject-matter is not the
nature of things, which is inexhaustible, but the understand-
ing which passes judgment upon the nature of things; and this
understanding, again, only in respect of its a priori knowledge. 
These a priori possessions of the understanding, since they
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have not to be sought for without, cannot remain hidden from
of our apprehending them in their completeness of judging
them. Still less may the reader here expect a critique of
books and systems of pure reason; we are concerned only with
the critique of the faculty of pure reason itself. Only in so far
as we build upon this foundation do we have a reliable touch-
stone for estimating the philosophical value of old and new
works in this field. Otherwise the unqualified historian or critic
is passing judgments upon the groundless assertions of others
by means of his own, which are equally groundless. 
Transcendental philosophy is only the idea of a science,
for which the critique of pure reason has to lay down the
complete architectonic plan. That is to say, it has to guaran-
tee, as following from principles, the completeness and cer-
tainty of the structure in all its parts. It is the system of
all principles of pure reason. And if this critique is not
itself to be entitled a transcendental philosophy, it is solely be-
cause, to be a complete system, it would also have to contain
an exhaustive analysis of the whole of a priori human know-
ledge. Our critique must, indeed, supply a complete enumera-
tion of all the fundamental concepts that go to constitute such
pure knowledge. But it is not required to give an exhaustive
analysis of these concepts, nor a complete review of those
that can be derived from them. Such a demand would be
unreasonable, partly because this analysis would not be
appropriate to our main purpose, inasmuch as there is no
such uncertainty in regard to analysis as we encounter in the
case of synthesis, for the sake of which alone our whole critique
is undertaken; and partly because it would be inconsistent
with the unity of our plan to assume responsibility for the com-
pleteness of such an analysis and derivation, when in view of our
purpose we can be excused from doing so. The analysis of these
a priori concepts, which later we shall have to enumerate, and
the derivation of other concepts from them, can easily, how-
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ever, be made complete when once they have been established
as exhausting the principles of synthesis, and if in this essen-
tial respect nothing be lacking in them. 
The critique of pure reason therefore will contain all that
is essential in transcendental philosophy. While it is the com-
plete idea of transcendental philosophy, it is not equivalent
to that latter science; for it carries the analysis only so far as
is requisite for the complete examination of knowledge which
is a priori and synthetic. 
What has chiefly to be kept in view in the division of such
a science, is that no concepts be allowed to enter which con-
tain in themselves anything empirical, or, in other words,
that it consist in knowledge wholly a priori. Accordingly,
although the highest principles and fundamental concepts of
morality are a priori knowledge, they have no place in tran-
scendental philosophy, because, although they do not lay at
the foundation of their precepts the concepts of pleasure
and pain, of the desires and inclinations, etc. , all of which
are of empirical origin, yet in the construction of a system
of pure morality these empirical concepts must necessarily
be brought into the concept of duty, as representing either a
hindrance, which we have to overcome, or an allurement, which
must not be made into a motive. Transcendental philosophy is
therefore a philosophy of pure and merely speculative reason. 
All that is practical, so far as it contains motives, relates to
feelings, and these belong to the empirical sources of know-
If we are to make a systematic division of the science
which we are engaged in presenting, it must have first a
doctrine of the elements, and secondly, a doctrine of the method
of pure reason. Each of these chief divisions will have its
subdivisions, but the grounds of these we are not yet in a
position to explain. By way of introduction or anticipation
we need only say that there are two stems of human know-
ledge, namely, sensibility and understanding, which perhaps
spring from a common, but to us unknown, root. Through the
former, objects are given to us; through the latter, they are
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thought. Now in so far as sensibility may be found to contain
a priori representations constituting the condition under
which objects are given to us, it will belong to transcendental
philosophy. And since the conditions under which alone the
objects of human knowledge are given must precede those
under which they are thought, the transcendental doctrine
of sensibility will constitute the first part of the science of the