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 Chrétien is Canada's Prime Minister)