I. OF THE DIFFERENCE BETWEEN PURE AND EMPIRICAL KNOWLEDGE
THAT all our knowledge begins with experience there can be no doubt.
For how is it possible that the faculty of cognition should be
awakened into exercise otherwise than by means of objects which affect
our senses, and partly of themselves produce representations, partly
rouse our powers of understanding into activity, to compare to
connect, or to separate these, and so to convert the raw material of
our sensuous impressions into a knowledge of objects, which is
called experience? In respect of time, therefore, no knowledge of ours
is antecedent to experience, but begins with it.
But, though all our knowledge begins with experience, it by no means
follows that all arises out of experience. For, on the contrary, it is
quite possible that our empirical knowledge is a compound of that
which we receive through impressions, and that which the faculty of
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.