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.
Pre-a priori thinkers included rationalists such as
Rene 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. (i.e. A triangle has three sides) A posteriori propositions will depend on external conditions, which may change in time, making the proposition false. (i.e. Jean Chr�tien is Canada's Prime Minister)
Major contemporary philosophers of a priori thought include Alfred Ayer, Roderick Chisholm and W.V.O. Quine.