Knowledge, Belief, and the A Priori

The notion of the a priori underwent several changes
since the time it came into existence in the Middle Ages.
Originally it had been used to mark a certain form of
argument, an argument that proceeds from what is prior to
what is later, from cause to effect: demonstratio procedens
ex causis ad effectum = demonstratio a priori. But this
changed with Kant, for whom it meant not a form of
argument but rather some special kind of knowledge (or
elements thereof), namely knowledge that (a) is independent
of particular experiences and (b) that makes experience
in general (Erfahrung überhaupt) possible. Tied up
with consciousness and the transcendental unity of
apperception, Kant"s understanding of the a priori was in
the spirit of his transcendental philosophy. But this
understanding changed again with the rise of analytic
philosophy, in which we still find the first characteristic but
not the second anymore. The idea of Erfahrung überhaupt
was given up, partly because one naturally wondered what
exactly this notion of experience in general, or experience
universally conceived, should be. Where should we get it
from, if not by way of abstraction and generalization from
individual cases of experience? And would this not make it
an empirical concept, so that the whole project of asking
for the conditions of its possibility would not lead us to the
kind of certainty, necessity and universality we expect from
a priori knowledge? There would be no guarantee that in
the future we would not make discoveries that would give
us new kinds of experiences or that would show us our
experiences in a new light. Thus, we would have to admit
that these experiences did not satisfy the conditions of
experience we had set up originally. The a priori conditions
would have to be revised.