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posteriori； that is； through experience。 Knowledge a priori is
either pure or impure。 Pure knowledge a priori is that with which no
empirical element is mixed up。 For example； the proposition； 〃Every
change has a cause；〃 is a proposition a priori； but impure； because
change is a conception which can only be derived from experience。
II。 The Human Intellect； even in an Unphilosophical State；
is in Possession of Certain Cognitions 〃a priori〃。
The question now is as to a criterion； by which we may securely
distinguish a pure from an empirical cognition。 Experience no doubt
teaches us that this or that object is constituted in such and such
a manner； but not that it could not possibly exist otherwise。 Now；
in the first place； if we have a proposition which contains the idea
of necessity in its very conception； it is a if； moreover； it is not
derived from any other proposition； unless from one equally
involving the idea of necessity； it is absolutely priori。 Secondly； an
empirical judgement never exhibits strict and absolute； but only
assumed and comparative universality （by induction）； therefore； the
most we can say is… so far as we have hitherto observed； there is no
exception to this or that rule。 If； on the other hand； a judgement
carries with it strict and absolute universality； that is； admits of
no possible exception； it is not derived from experience； but is valid
absolutely a priori。
Empirical universality is； therefore； only an arbitrary extension of
validity； from that which may be predicated of a proposition valid
in most cases； to that which is asserted of a proposition which
holds good in all； as； for example； in the affirmation； 〃All bodies
are heavy。〃 When； on the contrary； strict universality characterizes a
judgement； it necessarily indicates another peculiar source of
knowledge； namely； a faculty of cognition a priori。 Necessity and
strict universality； therefore； are infallible tests for
distinguishing pure from empirical knowledge； and are inseparably
connected with each other。 But as in the use of these criteria the
empirical limitation is sometimes more easily detected than the
contingency of the judgement； or the unlimited universality which we
attach to a judgement is often a more convincing proof than its
necessity； it may be advisable to use the criteria separately； each
being by itself infallible。
Now； that in the sphere of human cognition we have judgements
which are necessary； and in the strictest sense universal；
consequently pure a priori； it will be an easy matter to show。 If we
desire an example from the sciences； we need only take any proposition
in mathematics。 If we cast our eyes upon the commonest operations of
the understanding； the proposition； 〃Every change must have a
cause；〃 will amply serve our purpose。 In the latter case； indeed；
the conception of a cause so plainly involves the conception of a
necessity of connection with an effect； and of a strict universality
of the law； that the very notion of a cause would entirely
disappear； were we to derive it； like Hume； from a frequent
association of what happens with that which precedes； and the habit
thence originating of connecting representations… the necessity
inherent in the judgement being therefore merely subjective。
Besides； without seeking for such examples of principles existing a
priori in cognition； we might easily show that such principles are the
indispensable basis of the possibility of experience itself； and
consequently prove their existence a priori。 For whence could our
experience itself acquire certainty； if all the rules on which it
depends were themselves empirical； and consequently fortuitous？ No
one； therefore； can admit the validity of the use of such rules as
first principles。 But； for the present； we may content ourselves
with having established the fact； that we do possess and exercise a
faculty of pure a priori cognition； and； secondly； with having pointed
out the proper tests of such cognition； namely； universality and
necessity。
Not only in judgements； however； but even in conceptions； is an a
priori origin manifest。 For example； if we take away by degrees from
our conceptions of a body all that can be referred to mere sensuous
experience… colour； hardness or softness； weight； even
impenetrability… the body will then vanish； but the space which it
occupied still remains； and this it is utterly impossible to
annihilate in thought。 Again； if we take away； in like manner； from
our empirical conception of any object； corporeal or incorporeal；
all properties which mere experience has taught us to connect with it；
still we cannot think away those through which we cogitate it as
substance； or adhering to substance； although our conception of
substance is more determined than that of an object。 Compelled；
therefore； by that necessity with which the conception of substance
forces itself upon us； we must confess that it has its seat in our
faculty of cognition a priori。
III。 Philosophy stands in need of a Science which shall
Determine the Possibility； Principles； and Extent of
Human Knowledge 〃a priori〃
Of far more importance than all that has been above said； is the
consideration that certain of our cognitions rise completely above the
sphere of all possible experience； and by means of conceptions； to
which there exists in the whole extent of experience no
corresponding object； seem to extend the range of our judgements
beyond its bounds。 And just in this transcendental or supersensible
sphere； where experience affords us neither instruction nor
guidance； lie the investigations of reason； which； on account of their
importance； we consider far preferable to； and as having a far more
elevated aim than； all that the understanding can achieve within the
sphere of sensuous phenomena。 So high a value do we set upon these
investigations； that even at the risk of error； we persist in
following them out； and permit neither doubt nor disregard nor
indifference to restrain us from the pursuit。 These unavoidable
problems of mere pure reason are God； freedom （of will）； and
immortality。 The science which； with all its preliminaries； has for
its especial object the solution of these problems is named
metaphysics… a science which is at the very outset dogmatical； that
is； it confidently takes upon itself the execution of this task
without any previous investigation of the ability or inability of
reason for such an undertaking。
Now the safe ground of experience being thus abandoned； it seems
nevertheless natural that we should hesitate to erect a building
with the cognitions we possess； without knowing whence they come；
and on the strength of principles； the origin of which is
undiscovered。 Instead of thus trying to build without a foundation； it
is rather to be expected that we should long ago have put the
question； how the understanding can arrive at these a priori
co