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subtraction of certain elements；* 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 pel 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 pels 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。
*I do not here follow with exactness the history of the experimental
method； of which； indeed； the first steps are involved in some
obscurity。
We e now to metaphysics； a purely speculative science； which
occupies a pletely 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 es to a stand； when it attempts to
gain a priori the perception even of those laws which the most
mon experience confirms。 We find it pelled 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 batant 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 bee
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。*
*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 e 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
plete 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。*
*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 bines 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。*
*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 plete