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 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 plete
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 plete
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。*
*〃He considered nothing done； so long as anything remained to be
done。〃
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