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cognition supplies from itself （sensuous impressions giving merely the
occasion）； an addition which we cannot distinguish from the original
element given by sense； till long practice has made us attentive to；
and skilful in separating it。 It is； therefore； a question which
requires close investigation； and not to be answered at first sight；
whether there exists a knowledge altogether independent of experience；
and even of all sensuous impressions？ Knowledge of this kind is called
a priori； in contradistinction to empirical knowledge； which has its
sources a posteriori； that is； in experience。
But the expression； 〃a priori；〃 is not as yet definite enough
adequately to indicate the whole meaning of the question above
started。 For； in speaking of knowledge which has its sources in
experience； we are wont to say； that this or that may be known a
priori； because we do not derive this knowledge immediately from
experience； but from a general rule； which； however； we have itself
borrowed from experience。 Thus； if a man undermined his house； we say；
〃he might know a priori that it would have fallen；〃 that is； he needed
not to have waited for the experience that it did actually fall。 But
still； a priori； he could not know even this much。 For； that bodies
are heavy； and； consequently； that they fall when their supports are
taken away； must have been known to him previously； by means of
experience。
By the term 〃knowledge a priori；〃 therefore； we shall in the
sequel understand； not such as is independent of this or that kind
of experience； but such as is absolutely so of all experience。 Opposed
to this is empirical knowledge； or that which is possible only a
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 parative 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 monest 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