The Basics of Philosophy

a priori and a posteriori

_

These are terms primarily used to describe two species of propositional knowledge but also, derivatively, two classes of propositions or truths, namely, those that are knowable a priori and a posteriori respectively. Knowledge is said to be a priori when it does not depend for its authority upon the evidence of experience, and a posteriori when it does so depends.

Whether knowledge is a priori is quite a different question from whether it is innate. Mathematics provides the most often sited examples of a priori knowledge, but most of our mathematical knowledge is no doubt acquired through experience even though it is justifiable independent of experience. Kant and the others have held that a priori knowledge concerns only necessary truths while a posteriori knowledge concerns only contingent truths, but Kripke has challenged this assumption.

Source: The Oxford Guide to Philosophy, 2005