A PRIORI-A POSTERIORI (Latin prior—earlier, prior; posterior—later, posterior to)—from that which is earlier—from that which is later; in ordinary language: in advance, before the facts or knowledge of facts; in epistemological terms: opposite types of knowledge called a priori and a posteriori.
There are two different conceptions of what is a priori and a posteriori. (1) In the Middle Ages, a priori referred to a demonstration that advanced according to an ontological causal dependence (from causes to their effect); a posteriori referred to a demonstration that went in the opposite direction based on this relation of dependence (from an effect to its causes). A priori demonstration coincided with demonstratio propter quid, and a posteriori demonstration coincided with demonstratio quia. (2)When that which is general was treated as prior to what is individual, a priori was later used to describe reasoning that proceeded from the general to the particular (deduction), and a posteriori was used to describe reasoning from particulars to the general (induction). (3) A priori was a term for cognition (conceptions, judgments, and doctrines or sciences) acquired prior to any experience and independently of experience. Cognition was called a posteriori when it was acquired after experience and on the basis of experience.
A priori concepts are either innate to man, or experience merely elicits them from the human mind. Concepts are a posteriori when empirical knowledge or data performs an essential function in their genesis. Truths are a posteriori or a priori depending upon whether they are based on experience or are derived only from the reason. Some authors (especially Kant) divide a priori truths into analytic and synthetic, while others (e.g., the neopositivists) treat every a priori assertion as analytic. If the final premises in a science are all a priori judgments, it is called an a priori science (sometimes it is called a deductive science), and it is presented in opposition to a posteriori or empirical sciences which have some a posteriori judgments among their premises (apriorism, empiricism).
J. Lotz, Zum Problem des A priori, in; Mélanges Joseph Maréchal, Bru 1950, II 62–73; C. I. Lewis, A Pragmatic Conception of the A Priori, in: Readings in Philosophical Analysis (H. Feigl, W. Sellers), NY 1956; M. Dufrenne, La notion d’“a priori”, P 1959; H. Delius, Untersuchungen zur Problematik der sogenannten sythetischen Satze apriori, Go 1963; D. Locke, The Necessity of Analytic Truths, Philosophy 44 (1969), 12–32; J. Schepers, G. Tenell, HWP I 462–474.