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Philosophical thought
Pre-a priori thinkers included rationalists such as René 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 Chrétien is Canada's Prime Minister).
Major contemporary philosophers of a priori thought include Alfred Ayer, Roderick Chisholm and W.V.O. Quine.