THE CRITIQUE OF PURE REASON

By Immanuel Kant

Translated by J. M. D. Meiklejohn



PREFACE TO THE FIRST EDITION, 1781

Human reason, in one sphere of its cognition, is called upon to consider
questions, which it cannot decline, as they are presented by its own
nature, but which it cannot answer, as they transcend every faculty of
the mind.

It falls into this difficulty without any fault of its own. It begins
with principles, which cannot be dispensed with in the field of
experience, and the truth and sufficiency of which are, at the same
time, insured by experience. With these principles it rises, in
obedience to the laws of its own nature, to ever higher and more remote
conditions. But it quickly discovers that, in this way, its labours must
remain ever incomplete, because new questions never cease to present
themselves; and thus it finds itself compelled to have recourse to
principles which transcend the region of experience, while they are
regarded by common sense without distrust. It thus falls into confusion
and contradictions, from which it conjectures the presence of latent
errors, which, however, it is unable to discover, because the principles
it employs, transcending the limits of experience, cannot be tested
by that criterion. The arena of these endless contests is called
Metaphysic.

Time was, when she was the queen of all the sciences; and, if we take
the will for the deed, she certainly deserves, so far as regards the
high importance of her object-matter, this title of honour. Now, it is
the fashion of the time to heap contempt and scorn upon her; and the
matron mourns, forlorn and forsaken, like Hecuba:

    Modo maxima rerum,
    Tot generis, natisque potens...
    Nunc trahor exul, inops.
           --Ovid, Metamorphoses. xiii

At first, her government, under the administration of the dogmatists,
was an absolute despotism. But, as the legislative continued to show
traces of the ancient barbaric rule, her empire gradually broke up, and
intestine wars introduced the reign of anarchy; while the sceptics,
like nomadic tribes, who hate a permanent habitation and settled mode
of living, attacked from time to time those who had organized themselves
into civil communities. But their number was, very happily, small; and
thus they could not entirely put a stop to the exertions of those who
persisted in raising new edifices, although on no settled or uniform
plan. In recent times the hope dawned upon us of seeing those disputes
settled, and the legitimacy of her claims established by a kind of
physiology of the human understanding--that of the celebrated Locke. But
it was found that--although it was affirmed that this so-called queen
could not refer her descent to any higher source than that of common
experience, a circumstance which necessarily brought suspicion on
her claims--as this genealogy was incorrect, she persisted in the
advancement of her claims to sovereignty. Thus metaphysics necessarily
fell back into the antiquated and rotten constitution of dogmatism, and
again became obnoxious to the contempt from which efforts had been
made to save it. At present, as all methods, according to the general
persuasion, have been tried in vain, there reigns nought but weariness
and complete indifferentism--the mother of chaos and night in the
scientific world, but at the same time the source of, or at least the
prelude to, the re-creation and reinstallation of a science, when it has
fallen into confusion, obscurity, and disuse from ill directed effort.

For it is in reality vain to profess indifference in regard to such
inquiries, the object of which cannot be indifferent to humanity.
Besides, these pretended indifferentists, however much they may try to
disguise themselves by the assumption of a popular style and by changes
on the language of the schools, unavoidably fall into metaphysical
declarations and propositions, which they profess to regard with so much
contempt. At the same time, this indifference, which has arisen in the
world of science, and which relates to that kind of knowledge which
we should wish to see destroyed the last, is a phenomenon that well
deserves our attention and reflection. It is plainly not the effect of
the levity, but of the matured judgement* of the age, which refuses to
be any longer entertained with illusory knowledge, It is, in fact, a
call to reason, again to undertake the most laborious of all tasks--that
of self-examination, and to establish a tribunal, which may secure it
in its well-grounded claims, while it pronounces against all baseless
assumptions and pretensions, not in an arbitrary manner, but according
to its own eternal and unchangeable laws. This tribunal is nothing less
than the critical investigation of pure reason.

[*Footnote: We very often hear complaints of the shallowness of the
present age, and of the decay of profound science. But I do not think
that those which rest upon a secure foundation, such as mathematics,
physical science, etc., in the least deserve this reproach, but that
they rather maintain their ancient fame, and in the latter case, indeed,
far surpass it. The same would be the case with the other kinds of
cognition, if their principles were but firmly established. In the
absence of this security, indifference, doubt, and finally, severe
criticism are rather signs of a profound habit of thought. Our age
is the age of criticism, to which everything must be subjected. The
sacredness of religion, and the authority of legislation, are by many
regarded as grounds of exemption from the examination of this tribunal.
But, if they on they are exempted, they become the subjects of just
suspicion, and cannot lay claim to sincere respect, which reason accords
only to that which has stood the test of a free and public examination.]

I do not mean by this a criticism of books and systems, but a critical
inquiry into the faculty of reason, with reference to the cognitions
to which it strives to attain without the aid of experience; in other
words, the solution of the question regarding the possibility or
impossibility of metaphysics, and the determination of the origin, as
well as of the extent and limits of this science. All this must be done
on the basis of principles.

This path--the only one now remaining--has been entered upon by me; and
I flatter myself that I have, in this way, discovered the cause of--and
consequently the mode of removing--all the errors which have hitherto
set reason at variance with itself, in the sphere of non-empirical
thought. I have not returned an evasive answer to the questions of
reason, by alleging the inability and limitation of the faculties of the
mind; I have, on the contrary, examined them completely in the light
of principles, and, after having discovered the cause of the doubts and
contradictions into which reason fell, have solved them to its perfect
satisfaction. It is true, these questions have not been solved as
dogmatism, in its vain fancies and desires, had expected; for it can
only be satisfied by the exercise of magical arts, and of these I have
no knowledge. But neither do these come within the compass of our mental
powers; and it was the duty of philosophy to destroy the illusions which
had their origin in misconceptions, whatever darling hopes and valued
expectations may be ruined by its explanations. My chief aim in this
work has been thoroughness; and I make bold to say that there is not a
single metaphysical problem that does not find its solution, or at
least the key to its solution, here. Pure reason is a perfect unity; and
therefore, if the principle presented by it prove to be insufficient for
the solution of even a single one of those questions to which the very
nature of reason gives birth, we must reject it, as we could not be
perfectly certain of its sufficiency in the case of the others.

While I say this, I think I see upon the countenance of the reader signs
of dissatisfaction mingled with contempt, when he hears declarations
which sound so boastful and extravagant; and yet they are beyond
comparison more moderate than those advanced by the commonest author of
the commonest philosophical programme, in which the dogmatist professes
to demonstrate the simple nature of the soul, or the necessity of a
primal being. Such a dogmatist promises to extend human knowledge beyond
the limits of possible experience; while I humbly confess that this
is completely beyond my power. Instead of any such attempt, I confine
myself to the examination of reason alone and its pure thought; and I do
not need to seek far for the sum-total of its cognition, because it
has its seat in my own mind. Besides, common logic presents me with
a complete and systematic catalogue of all the simple operations of
reason; and it is my task to answer the question how far reason can go,
without the material presented and the aid furnished by experience.

So much for the completeness and thoroughness necessary in the execution
of the present task. The aims set before us are not arbitrarily
proposed, but are imposed upon us by the nature of cognition itself.

The above remarks relate to the matter of our critical inquiry. As
regards the form, there are two indispensable conditions, which any one
who undertakes so difficult a task as that of a critique of pure reason,
is bound to fulfil. These conditions are certitude and clearness.

As regards certitude, I have fully convinced myself that, in this sphere
of thought, opinion is perfectly inadmissible, and that everything which
bears the least semblance of an hypothesis must be excluded, as of no
value in such discussions. For it is a necessary condition of every
cognition that is to be established upon a priori grounds that it shall
be held to be absolutely necessary; much more is this the case with an
attempt to determine all pure a priori cognition, and to furnish the
standard--and consequently an example--of all apodeictic (philosophical)
certitude. Whether I have succeeded in what I professed to do, it is for
the reader to determine; it is the author's business merely to adduce
grounds and reasons, without determining what influence these ought to
have on the mind of his judges. But, lest anything he may have said may
become the innocent cause of doubt in their minds, or tend to weaken the
effect which his arguments might otherwise produce--he may be allowed
to point out those passages which may occasion mistrust or difficulty,
although these do not concern the main purpose of the present work. He
does this solely with the view of removing from the mind of the reader
any doubts which might affect his judgement of the work as a whole, and
in regard to its ultimate aim.

I know no investigations more necessary for a full insight into the
nature of the faculty which we call understanding, and at the same time
for the determination of the rules and limits of its use, than those
undertaken in the second chapter of the "Transcendental Analytic," under
the title of "Deduction of the Pure Conceptions of the Understanding";
and they have also cost me by far the greatest labour--labour which, I
hope, will not remain uncompensated. The view there taken, which goes
somewhat deeply into the subject, has two sides, The one relates to the
objects of the pure understanding, and is intended to demonstrate and
to render comprehensible the objective validity of its a priori
conceptions; and it forms for this reason an essential part of the
Critique. The other considers the pure understanding itself, its
possibility and its powers of cognition--that is, from a subjective
point of view; and, although this exposition is of great importance, it
does not belong essentially to the main purpose of the work, because the
grand question is what and how much can reason and understanding, apart
from experience, cognize, and not, how is the faculty of thought itself
possible? As the latter is an inquiry into the cause of a given effect,
and has thus in it some semblance of an hypothesis (although, as I shall
show on another occasion, this is really not the fact), it would seem
that, in the present instance, I had allowed myself to enounce a mere
opinion, and that the reader must therefore be at liberty to hold
a different opinion. But I beg to remind him that, if my subjective
deduction does not produce in his mind the conviction of its certitude
at which I aimed, the objective deduction, with which alone the present
work is properly concerned, is in every respect satisfactory.

As regards clearness, the reader has a right to demand, in the first
place, discursive or logical clearness, that is, on the basis of
conceptions, and, secondly, intuitive or aesthetic clearness, by means
of intuitions, that is, by examples or other modes of illustration
in concreto. I have done what I could for the first kind of
intelligibility. This was essential to my purpose; and it thus became
the accidental cause of my inability to do complete justice to the
second requirement. I have been almost always at a loss, during the
progress of this work, how to settle this question. Examples and
illustrations always appeared to me necessary, and, in the first sketch
of the Critique, naturally fell into their proper places. But I very
soon became aware of the magnitude of my task, and the numerous problems
with which I should be engaged; and, as I perceived that this critical
investigation would, even if delivered in the driest scholastic manner,
be far from being brief, I found it unadvisable to enlarge it still more
with examples and explanations, which are necessary only from a popular
point of view. I was induced to take this course from the consideration
also that the present work is not intended for popular use, that those
devoted to science do not require such helps, although they are always
acceptable, and that they would have materially interfered with my
present purpose. Abbe Terrasson remarks with great justice that, if we
estimate the size of a work, not from the number of its pages, but from
the time which we require to make ourselves master of it, it may be said
of many a book that it would be much shorter, if it were not so short.
On the other hand, as regards the comprehensibility of a system of
speculative cognition, connected under a single principle, we may say
with equal justice: many a book would have been much clearer, if it had
not been intended to be so very clear. For explanations and examples,
and other helps to intelligibility, aid us in the comprehension of
parts, but they distract the attention, dissipate the mental power of
the reader, and stand in the way of his forming a clear conception of
the whole; as he cannot attain soon enough to a survey of the system,
and the colouring and embellishments bestowed upon it prevent his
observing its articulation or organization--which is the most important
consideration with him, when he comes to judge of its unity and
stability.

The reader must naturally have a strong inducement to co-operate
with the present author, if he has formed the intention of erecting a
complete and solid edifice of metaphysical science, according to the
plan now laid before him. Metaphysics, as here represented, is the only
science which admits of completion--and with little labour, if it
is united, in a short time; so that nothing will be left to
future generations except the task of illustrating and applying it
didactically. For this science is nothing more than the inventory of all
that is given us by pure reason, systematically arranged. Nothing can
escape our notice; for what reason produces from itself cannot lie
concealed, but must be brought to the light by reason itself, so soon
as we have discovered the common principle of the ideas we seek. The
perfect unity of this kind of cognitions, which are based upon pure
conceptions, and uninfluenced by any empirical element, or any peculiar
intuition leading to determinate experience, renders this completeness
not only practicable, but also necessary.

    Tecum habita, et noris quam sit tibi curta supellex.
           --Persius. Satirae iv. 52.

Such a system of pure speculative reason I hope to be able to publish
under the title of Metaphysic of Nature*. The content of this work
(which will not be half so long) will be very much richer than that
of the present Critique, which has to discover the sources of this
cognition and expose the conditions of its possibility, and at the same
time to clear and level a fit foundation for the scientific edifice. In
the present work, I look for the patient hearing and the impartiality
of a judge; in the other, for the good-will and assistance of a
co-labourer. For, however complete the list of principles for this
system may be in the Critique, the correctness of the system requires
that no deduced conceptions should be absent. These cannot be presented
a priori, but must be gradually discovered; and, while the synthesis of
conceptions has been fully exhausted in the Critique, it is necessary
that, in the proposed work, the same should be the case with their
analysis. But this will be rather an amusement than a labour.

     [*Footnote: In contradistinction to the Metaphysic of
     Ethics. This work was never published.]



PREFACE TO THE SECOND EDITION, 1787

Whether the treatment of that portion of our knowledge which lies within
the province of pure reason advances with that undeviating certainty
which characterizes the progress of science, we shall be at no loss to
determine. If we find those who are engaged in metaphysical pursuits,
unable to come to an understanding as to the method which they ought
to follow; if we find them, after the most elaborate preparations,
invariably brought to a stand before the goal is reached, and compelled
to retrace their steps and strike into fresh paths, we may then feel
quite sure that they are far from having attained to the certainty of
scientific progress and may rather be said to be merely groping about in
the dark. In these circumstances we shall render an important service to
reason if we succeed in simply indicating the path along which it must
travel, in order to arrive at any results--even if it should be found
necessary to abandon many of those aims which, without reflection, have
been proposed for its attainment.

That logic has advanced in this sure course, even from the earliest
times, is apparent from the fact that, since Aristotle, it has been
unable to advance a step and, thus, to all appearance has reached its
completion. For, if some of the moderns have thought to enlarge its
domain by introducing psychological discussions on the mental faculties,
such as imagination and wit, metaphysical, discussions on the origin
of knowledge and the different kinds of certitude, according to
the difference of the objects (idealism, scepticism, and so on), or
anthropological discussions on prejudices, their causes and remedies:
this attempt, on the part of these authors, only shows their ignorance
of the peculiar nature of logical science. We do not enlarge but
disfigure the sciences when we lose sight of their respective limits and
allow them to run into one another. Now logic is enclosed within limits
which admit of perfectly clear definition; it is a science which has for
its object nothing but the exposition and proof of the formal laws of
all thought, whether it be a priori or empirical, whatever be its
origin or its object, and whatever the difficulties--natural or
accidental--which it encounters in the human mind.

The early success of logic must be attributed exclusively to the
narrowness of its field, in which abstraction may, or rather must,
be made of all the objects of cognition with their characteristic
distinctions, and in which the understanding has only to deal with
itself and with its own forms. It is, obviously, a much more difficult
task for reason to strike into the sure path of science, where it has to
deal not simply with itself, but with objects external to itself. Hence,
logic is properly only a propaedeutic--forms, as it were, the vestibule
of the sciences; and while it is necessary to enable us to form a
correct judgement with regard to the various branches of knowledge,
still the acquisition of real, substantive knowledge is to be sought
only in the sciences properly so called, that is, in the objective
sciences.

Now these sciences, if they can be termed rational at all, must contain
elements of a priori cognition, and this cognition may stand in a
twofold relation to its object. Either it may have to determine the
conception of the object--which must be supplied extraneously, or it
may have to establish its reality. The former is theoretical, the latter
practical, rational cognition. In both, the pure or a priori element
must be treated first, and must be carefully distinguished from that
which is supplied from other sources. Any other method can only lead to
irremediable confusion.

Mathematics and physics are the two theoretical sciences which have to
determine their objects a priori. The former is purely a priori, the
latter is partially so, but is also dependent on other sources of
cognition.

In the earliest times of which history affords us any record,
mathematics had already entered on the sure course of science, among
that wonderful nation, the Greeks. Still it is not to be supposed that
it was as easy for this science to strike into, or rather to construct
for itself, that royal road, as it was for logic, in which reason has
only to deal with itself. On the contrary, I believe that it must
have remained long--chiefly among the Egyptians--in the stage of
blind groping after its true aims and destination, and that it was
revolutionized by the happy idea of one man, who struck out and
determined for all time the path which this science must follow,
and which admits of an indefinite advancement. The history of this
intellectual revolution--much more important in its results than the
discovery of the passage round the celebrated Cape of Good Hope--and of
its author, has not been preserved. But Diogenes Laertius, in naming
the supposed discoverer of some of the simplest elements of geometrical
demonstration--elements which, according to the ordinary opinion, do not
even require to be proved--makes it apparent that the change introduced
by the first indication of this new path, must have seemed of the utmost
importance to the mathematicians of that age, and it has thus been
secured against the chance of oblivion. A new light must have flashed on
the mind of the first man (Thales, or whatever may have been his name)
who demonstrated the properties of the isosceles triangle. For he found
that it was not sufficient to meditate on the figure, as it lay before
his eyes, or the conception of it, as it existed in his mind, and thus
endeavour to get at the knowledge of its properties, but that it was
necessary to produce these properties, as it were, by a positive a
priori construction; and that, in order to arrive with certainty at
a priori cognition, he must not attribute to the object any other
properties than those which necessarily followed from that which he had
himself, in accordance with his conception, placed in the object.

A much longer period elapsed before physics entered on the highway of
science. For it is only about a century and a half since the wise Bacon
gave a new direction to physical studies, or rather--as others were
already on the right track--imparted fresh vigour to the pursuit of
this new direction. Here, too, as in the case of mathematics, we find
evidence of a rapid intellectual revolution. In the remarks which follow
I shall confine myself to the empirical side of natural science.

When Galilei experimented with balls of a definite weight on the
inclined plane, when Torricelli caused the air to sustain a weight which
he had calculated beforehand to be equal to that of a definite column of
water, or when Stahl, at a later period, converted metals into lime, and
reconverted lime into metal, by the addition and subtraction of certain
elements; [Footnote: I do not here follow with exactness the history of
the experimental method, of which, indeed, the first steps are involved
in some obscurity.] a light broke upon all natural philosophers. They
learned that reason only perceives that which it produces after its
own design; that it must not be content to follow, as it were, in the
leading-strings of nature, but must proceed in advance with principles
of judgement according to unvarying laws, and compel nature to reply
its questions. For accidental observations, made according to no
preconceived plan, cannot be united under a necessary law. But it is
this that reason seeks for and requires. It is only the principles of
reason which can give to concordant phenomena the validity of laws, and
it is only when experiment is directed by these rational principles that
it can have any real utility. Reason must approach nature with the view,
indeed, of receiving information from it, not, however, in the character
of a pupil, who listens to all that his master chooses to tell him,
but in that of a judge, who compels the witnesses to reply to those
questions which he himself thinks fit to propose. To this single idea
must the revolution be ascribed, by which, after groping in the dark for
so many centuries, natural science was at length conducted into the path
of certain progress.

We come now to metaphysics, a purely speculative science, which occupies
a completely isolated position and is entirely independent of the
teachings of experience. It deals with mere conceptions--not, like
mathematics, with conceptions applied to intuition--and in it, reason is
the pupil of itself alone. It is the oldest of the sciences, and would
still survive, even if all the rest were swallowed up in the abyss of
an all-destroying barbarism. But it has not yet had the good fortune to
attain to the sure scientific method. This will be apparent; if we
apply the tests which we proposed at the outset. We find that reason
perpetually comes to a stand, when it attempts to gain a priori the
perception even of those laws which the most common experience confirms.
We find it compelled to retrace its steps in innumerable instances, and
to abandon the path on which it had entered, because this does not
lead to the desired result. We find, too, that those who are engaged in
metaphysical pursuits are far from being able to agree among themselves,
but that, on the contrary, this science appears to furnish an arena
specially adapted for the display of skill or the exercise of strength
in mock-contests--a field in which no combatant ever yet succeeded in
gaining an inch of ground, in which, at least, no victory was ever yet
crowned with permanent possession.

This leads us to inquire why it is that, in metaphysics, the sure path
of science has not hitherto been found. Shall we suppose that it is
impossible to discover it? Why then should nature have visited our
reason with restless aspirations after it, as if it were one of our
weightiest concerns? Nay, more, how little cause should we have to place
confidence in our reason, if it abandons us in a matter about which,
most of all, we desire to know the truth--and not only so, but even
allures us to the pursuit of vain phantoms, only to betray us in the
end? Or, if the path has only hitherto been missed, what indications do
we possess to guide us in a renewed investigation, and to enable us to
hope for greater success than has fallen to the lot of our predecessors?

It appears to me that the examples of mathematics and natural
philosophy, which, as we have seen, were brought into their present
condition by a sudden revolution, are sufficiently remarkable to fix our
attention on the essential circumstances of the change which has proved
so advantageous to them, and to induce us to make the experiment of
imitating them, so far as the analogy which, as rational sciences, they
bear to metaphysics may permit. It has hitherto been assumed that our
cognition must conform to the objects; but all attempts to ascertain
anything about these objects a priori, by means of conceptions, and thus
to extend the range of our knowledge, have been rendered abortive by
this assumption. Let us then make the experiment whether we may not
be more successful in metaphysics, if we assume that the objects must
conform to our cognition. This appears, at all events, to accord better
with the possibility of our gaining the end we have in view, that is to
say, of arriving at the cognition of objects a priori, of determining
something with respect to these objects, before they are given to us. We
here propose to do just what Copernicus did in attempting to explain
the celestial movements. When he found that he could make no progress by
assuming that all the heavenly bodies revolved round the spectator,
he reversed the process, and tried the experiment of assuming that the
spectator revolved, while the stars remained at rest. We may make
the same experiment with regard to the intuition of objects. If the
intuition must conform to the nature of the objects, I do not see how
we can know anything of them a priori. If, on the other hand, the object
conforms to the nature of our faculty of intuition, I can then easily
conceive the possibility of such an a priori knowledge. Now as I cannot
rest in the mere intuitions, but--if they are to become cognitions--must
refer them, as representations, to something, as object, and must
determine the latter by means of the former, here again there are two
courses open to me. Either, first, I may assume that the conceptions,
by which I effect this determination, conform to the object--and in this
case I am reduced to the same perplexity as before; or secondly, I may
assume that the objects, or, which is the same thing, that experience,
in which alone as given objects they are cognized, conform to my
conceptions--and then I am at no loss how to proceed. For experience
itself is a mode of cognition which requires understanding. Before
objects, are given to me, that is, a priori, I must presuppose in myself
laws of the understanding which are expressed in conceptions a
priori. To these conceptions, then, all the objects of experience must
necessarily conform. Now there are objects which reason thinks, and
that necessarily, but which cannot be given in experience, or, at least,
cannot be given so as reason thinks them. The attempt to think these
objects will hereafter furnish an excellent test of the new method of
thought which we have adopted, and which is based on the principle that
we only cognize in things a priori that which we ourselves place in
them.*

     [*Footnote: This method, accordingly, which we have borrowed
     from the natural philosopher, consists in seeking for the
     elements of pure reason in that which admits of confirmation
     or refutation by experiment. Now the propositions of pure
     reason, especially when they transcend the limits of
     possible experience, do not admit of our making any
     experiment with their objects, as in natural science. Hence,
     with regard to those conceptions and principles which we
     assume a priori, our only course ill be to view them from
     two different sides. We must regard one and the same
     conception, on the one hand, in relation to experience as an
     object of the senses and of the understanding, on the other
     hand, in relation to reason, isolated and transcending the
     limits of experience, as an object of mere thought. Now if
     we find that, when we regard things from this double point
     of view, the result is in harmony with the principle of pure
     reason, but that, when we regard them from a single point of
     view, reason is involved in self-contradiction, then the
     experiment will establish the correctness of this
     distinction.]

This attempt succeeds as well as we could desire, and promises to
metaphysics, in its first part--that is, where it is occupied with
conceptions a priori, of which the corresponding objects may be given in
experience--the certain course of science. For by this new method we are
enabled perfectly to explain the possibility of a priori cognition, and,
what is more, to demonstrate satisfactorily the laws which lie a
priori at the foundation of nature, as the sum of the objects of
experience--neither of which was possible according to the procedure
hitherto followed. But from this deduction of the faculty of a priori
cognition in the first part of metaphysics, we derive a surprising
result, and one which, to all appearance, militates against the great
end of metaphysics, as treated in the second part. For we come to the
conclusion that our faculty of cognition is unable to transcend the
limits of possible experience; and yet this is precisely the most
essential object of this science. The estimate of our rational cognition
a priori at which we arrive is that it has only to do with phenomena,
and that things in themselves, while possessing a real existence,
lie beyond its sphere. Here we are enabled to put the justice of this
estimate to the test. For that which of necessity impels us to transcend
the limits of experience and of all phenomena is the unconditioned,
which reason absolutely requires in things as they are in themselves,
in order to complete the series of conditions. Now, if it appears that
when, on the one hand, we assume that our cognition conforms to its
objects as things in themselves, the unconditioned cannot be thought
without contradiction, and that when, on the other hand, we assume that
our representation of things as they are given to us, does not conform
to these things as they are in themselves, but that these objects, as
phenomena, conform to our mode of representation, the contradiction
disappears: we shall then be convinced of the truth of that which we
began by assuming for the sake of experiment; we may look upon it as
established that the unconditioned does not lie in things as we
know them, or as they are given to us, but in things as they are in
themselves, beyond the range of our cognition.*

     [*Footnote: This experiment of pure reason has a great
     similarity to that of the chemists, which they term the
     experiment of reduction, or, more usually, the synthetic
     process. The analysis of the metaphysician separates pure
     cognition a priori into two heterogeneous elements, viz.,
     the cognition of things as phenomena, and of things in
     themselves. Dialectic combines these again into harmony with
     the necessary rational idea of the unconditioned, and finds
     that this harmony never results except through the above
     distinction, which is, therefore, concluded to be just.]

But, after we have thus denied the power of speculative reason to make
any progress in the sphere of the supersensible, it still remains for
our consideration whether data do not exist in practical cognition
which may enable us to determine the transcendent conception of the
unconditioned, to rise beyond the limits of all possible experience
from a practical point of view, and thus to satisfy the great ends of
metaphysics. Speculative reason has thus, at least, made room for such
an extension of our knowledge: and, if it must leave this space vacant,
still it does not rob us of the liberty to fill it up, if we can,
by means of practical data--nay, it even challenges us to make the
attempt.*

     [*Footnote: So the central laws of the movements of the
     heavenly bodies established the truth of that which
     Copernicus, first, assumed only as a hypothesis, and, at the
     same time, brought to light that invisible force (Newtonian
     attraction) which holds the universe together. The latter
     would have remained forever undiscovered, if Copernicus had
     not ventured on the experiment--contrary to the senses but
     still just--of looking for the observed movements not in the
     heavenly bodies, but in the spectator. In this Preface I
     treat the new metaphysical method as a hypothesis with the
     view of rendering apparent the first attempts at such a
     change of method, which are always hypothetical. But in the
     Critique itself it will be demonstrated, not hypothetically,
     but apodeictically, from the nature of our representations
     of space and time, and from the elementary conceptions of
     the understanding.]


This attempt to introduce a complete revolution in the procedure
of metaphysics, after the example of the geometricians and natural
philosophers, constitutes the aim of the Critique of Pure Speculative
Reason. It is a treatise on the method to be followed, not a system of
the science itself. But, at the same time, it marks out and defines both
the external boundaries and the internal structure of this science.
For pure speculative reason has this peculiarity, that, in choosing the
various objects of thought, it is able to define the limits of its own
faculties, and even to give a complete enumeration of the possible
modes of proposing problems to itself, and thus to sketch out the entire
system of metaphysics. For, on the one hand, in cognition a priori,
nothing must be attributed to the objects but what the thinking subject
derives from itself; and, on the other hand, reason is, in regard to
the principles of cognition, a perfectly distinct, independent unity, in
which, as in an organized body, every member exists for the sake of
the others, and all for the sake of each, so that no principle can be
viewed, with safety, in one relationship, unless it is, at the same
time, viewed in relation to the total use of pure reason. Hence, too,
metaphysics has this singular advantage--an advantage which falls to the
lot of no other science which has to do with objects--that, if once it
is conducted into the sure path of science, by means of this criticism,
it can then take in the whole sphere of its cognitions, and can thus
complete its work, and leave it for the use of posterity, as a capital
which can never receive fresh accessions. For metaphysics has to deal
only with principles and with the limitations of its own employment as
determined by these principles. To this perfection it is, therefore,
bound, as the fundamental science, to attain, and to it the maxim may
justly be applied:

    Nil actum reputans, si quid superesset agendum.

But, it will be asked, what kind of a treasure is this that we propose
to bequeath to posterity? What is the real value of this system of
metaphysics, purified by criticism, and thereby reduced to a permanent
condition? A cursory view of the present work will lead to the
supposition that its use is merely negative, that it only serves to
warn us against venturing, with speculative reason, beyond the limits
of experience. This is, in fact, its primary use. But this, at once,
assumes a positive value, when we observe that the principles with which
speculative reason endeavours to transcend its limits lead inevitably,
not to the extension, but to the contraction of the use of reason,
inasmuch as they threaten to extend the limits of sensibility, which
is their proper sphere, over the entire realm of thought and, thus,
to supplant the pure (practical) use of reason. So far, then, as this
criticism is occupied in confining speculative reason within its proper
bounds, it is only negative; but, inasmuch as it thereby, at the same
time, removes an obstacle which impedes and even threatens to destroy
the use of practical reason, it possesses a positive and very important
value. In order to admit this, we have only to be convinced that there
is an absolutely necessary use of pure reason--the moral use--in which
it inevitably transcends the limits of sensibility, without the aid
of speculation, requiring only to be insured against the effects of a
speculation which would involve it in contradiction with itself. To deny
the positive advantage of the service which this criticism renders
us would be as absurd as to maintain that the system of police is
productive of no positive benefit, since its main business is to prevent
the violence which citizen has to apprehend from citizen, that so each
may pursue his vocation in peace and security. That space and time are
only forms of sensible intuition, and hence are only conditions of the
existence of things as phenomena; that, moreover, we have no conceptions
of the understanding, and, consequently, no elements for the cognition
of things, except in so far as a corresponding intuition can be given
to these conceptions; that, accordingly, we can have no cognition of
an object, as a thing in itself, but only as an object of sensible
intuition, that is, as phenomenon--all this is proved in the analytical
part of the Critique; and from this the limitation of all possible
speculative cognition to the mere objects of experience, follows as a
necessary result. At the same time, it must be carefully borne in mind
that, while we surrender the power of cognizing, we still reserve the
power of thinking objects, as things in themselves.* For, otherwise,
we should require to affirm the existence of an appearance, without
something that appears--which would be absurd. Now let us suppose, for a
moment, that we had not undertaken this criticism and, accordingly,
had not drawn the necessary distinction between things as objects
of experience and things as they are in themselves. The principle of
causality, and, by consequence, the mechanism of nature as determined by
causality, would then have absolute validity in relation to all things
as efficient causes. I should then be unable to assert, with regard to
one and the same being, e.g., the human soul, that its will is free, and
yet, at the same time, subject to natural necessity, that is,