A priori / A posteriori — An epistemological distinction that classifies knowledge according to its relation to experience. The terms come from Latin — a priori means “from what comes before” and a posteriori “from what comes after” — and circulated in medieval scholastic logic before acquiring their modern sense. A priori knowledge is knowledge whose justification is independent of sensory experience: it rests on reason alone, as with the truths of mathematics and the principles of logic. A posteriori (or empirical) knowledge depends on observation and experience, such as knowing that water boils at a certain temperature or that snow is white. Its distinguishing marks are usually taken to be necessity and universality: what is known a priori holds necessarily and without exception, whereas empirical knowledge remains contingent and particular. It was Immanuel Kant who gave the distinction its classic formulation in the Critique of Pure Reason (1781). Against the simple division between analytic judgments (whose predicate is already contained in the subject) and synthetic ones (which add new information), Kant argued for the existence of synthetic a priori judgments: statements that extend knowledge and yet are universally valid without recourse to experience — such as “7 + 5 = 12” or “the straight line is the shortest path between two points.” For Kant, the very possibility of mathematics and physics depended on such judgments. The distinction is thus closely bound up with the analytic/synthetic pair and with the notion of empirical knowledge, and it remains central to the theory of knowledge.
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