Distinction among judgments, propositions, concepts, ideas, arguments, or kinds of knowledge. In each case, the a priori is taken to be independent of sensory experience, which the a posteriori presupposes. An a priori argument, then, is taken to reason deductively from abstract general premises, while an a posteriori argument relies upon specific information derived from sense perception. The necessary truth of an a priori proposition can be determined by reason alone, but the contingent truth of an a posteriori proposition can be discovered only by reference to some matter of fact. Thus, for example:

"3 + 4 = 7." is known a priori.

"Chicago is located on the shore of Lake Michigan." is known a posteriori.

Rationalists typically emphasize the importance of a priori ideas and arguments in establishing genuine knowledge on a firm foundation. Kant argued that synthetic a priori judgments are preconditions for any experience and thus provide a basis for mathematical and scientific knowledge. Empiricists, on the other hand, usually hold that all a priori propositions are merely analytic, so that we must rely on a posteriori propositions for significant information about the world. Kripke challenges even the identification of this distinction with that between the necessary and the contingent.