A PRIORI/A POSTERIORI

A contrast first between propositions. A proposition is knowable a priori if it can be known without experience of the specific course of events in the actual world. It may, however, be allowed that some experience is required to acquire the concepts involved in an a priori proposition. Something is knowable only a posteriori if it cannot be known a priori . The distinction gives one of the fundamental problem areas of epistemology .The category of a priori propositions is highly controversial, since it is not clear how pure thought, unaided by experience, can give rise to any knowledge at all, and it has always been a concern of empiricism to deny that it can. The two great areas in which it seems to do so are logic and mathematics, so empiricists have commonly tried to show either that these are not areas of real, substantive knowledge, or that in spite of appearances the knowledge that we have in these areas is actually dependent on experience. The former line tries to show that all a priori propositions are in some sense trivial, or analytic, or matters of notation or conventions of language. The latter approach is particularly associated with Quine, who denies any significant split between propositions traditionally thought of as a priori, and other deeply entrenched beliefs that occur in our overall view of the world.

Another contested category is that of a priori concepts, supposed to be concepts that cannot be ‘derived’ from experience, but which are presupposed in any mode of thought about the world: time, substance, causation, number, and the self are candidates. The need for such concepts, and the nature of the substantive a priori knowledge to which they give rise, is the central concern of Kant's Critique of Pure Reason .