A PRIORI and A POSTERIORI. - Technical expressions of philosophical language, of varied meaning. Their origin goes back to Aristotle, who in the Analyt. Poster., I, 2, 71 b, 33 had distinguished things that are first and better known, priora and notiora, either with regard to our knowledge (πρὸς ἡμᾶς), or absolutely in themselves (τῇ φύσει, ἀπλῶς): priora and notiora quoad nos are sensible and singular things, while priora and notiora quoad se are universals. From this Aristotelian distinction derived the common usage of medieval Scholasticism, which gave to the a pr. and a post. an exclusively logical and methodological meaning in relation to a particular kind of reasoning: the argument that goes from the general to the particular, from genus to species, from cause to effect, from essence to properties, is a pr., while the argument that takes the opposite path, from the particular to the universal, from effect to cause, from properties to essence, is a post. This meaning of the a pr. and a post., already present in St. Thomas, who denies the possibility of an a pr. demonstration of the existence of God (Sum. Theol., 1ᵃ, q. 2, a. 2), is more explicitly formulated by Albert of Saxony (cf. K. Prantl, Gesch. der Logik, IV, Leipzig 1927, p. 78).
In modern philosophy, upon the medieval logical sense, which had not entirely fallen away, there was superimposed, and it prevailed, a rather epistemological meaning according to which a pr. are understood to be all cognitions that, at least in some way, do not depend on experience, and a post. those that derive from experience. This sense is already found in Hume (Treatise of Human Nature, I, p. iv), and in Leibniz (Nuovi Saggi, IV, 9, 2; Ital. trans., Bari 1926, II, p. 194). It was chiefly Kant who contributed to making the modern meaning universal; in his system the a pr. acquires a central position. Kant distinguishes empirical cognitions, proceeding from experience, and a pr. cognitions, which, understood in a rigorous sense, are absolutely independent of all experience, indeed presupposed to experience itself (Critica della ragion pura, Bari 1924, p. 39). He then subdivides a pr. cognitions into pure and non-pure, according to whether or not something empirical is intermixed with them. The distinguishing characteristics of a pr. cognitions, in no way derivable from experience, are absolute necessity and universality (ibid., pp. 39-40).
The Kantian sense of the a pr. has remained, both in its more rigorously technical expression and in a broader and more current reference, extended to everything that can be known by reason alone independently of this or that experience; in this sense the expressions have also entered common language, which distinguishes judgments, reasonings, methods a pr. and a post.