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