A priori
A priori knowledge is propositional knowledge that can be had without experience. It is usually contrasted with a posteriori knowledge, which requires experience. Mathematics and logic are usually considered a priori disciplines. The natural and social sciences are usually considered a posteriori disciplines.
Philosophical thought
Pre-a priori thinkers included rationalists such as RenÃÂé Descartes and Gottfried Leibniz, who argued that knowledge is gained through reason, not experience. However, modern a priori thought began with Immanuel Kant who brought up the contemporary distinction between a priori and a posteriori knowledge.
He argues that propositions known a priori are necessarily true, while propositions known a posteriori are contingent, because a priori knowledge has always been true, according to Kant (e.g. two plus two equals four). A posteriori propositions will depend on external conditions, which may change in time, making the proposition false (e.g. Jean ChrÃÂétien is Canada's Prime Minister).
Saul Kripke argues in Naming and Necessity, contra Kant, that aprioricity is an epistemological property, and should not be conflated with the separate, metaphysical matter of necessity. In support of this argument he offers several pleas to intuition: "Hesperus is Phosphorus" is necessary if true (see rigid designation), but known a posteriori; while, on the other hand, (of the bar in Paris that formerly served as the standard for the meter), "That bar is one meter long" is contingent (it could have shrunk, etc.) but known a priori.
Major contemporary philosophers of a priori thought include Alfred Ayer, Roderick Chisholm and W.V.O. Quine.