A PRIORI and A POSTERIORI. — Technical expressions of philosophical language, with varied meanings. Their origin goes back to Aristotle, who in the Analyt. Poster., I, 2, 71 b, 33 had distinguished things that are prior 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, whereas priora and notiora quoad se are universals. From this Aristotelian distinction arose the common usage of medieval Scholasticism, which gave a pr. and a post. an exclusively logical and methodological meaning in relation to a particular kind of reasoning: an argument is a pr. when it proceeds from the general to the particular, from genus to species, from cause to effect, from essence to properties, whereas an argument is a post. when it follows the opposite route, from the particular to the universal, from effect to cause, from properties to essence. This meaning of a pr. and a post., already present in s. Tommaso, who denies the possibility of an a pr. demonstration of the existence of God (Sum. Theol., 1ᵃ, q. 2, a. 2), was formulated more explicitly by Alberto di Sassonia (cf. K. Prantl, Gesch. der Logik, IV, Lipsia 1927, p. 78).
In modern philosophy, a rather epistemological meaning was superimposed upon, and prevailed over, the medieval logical sense, which did not entirely disappear: according to this meaning, all cognitions that, at least in some way, do not depend on experience are understood as a pr., and those derived from experience as a post. This sense is already found in Hume (Treatise of Human Nature, I, p. iv), and in Leibniz (Nuovi Saggi, IV, 9, 2; Italian trans., Bari 1926, II, p. 194). Kant contributed principally to making the modern meaning universal, since in his system the a pr. acquires a central position. Kant distinguishes empirical cognitions, deriving from experience, from a pr. cognitions, which, understood in the strict sense, are absolutely independent of every experience, indeed presupposed by experience itself (Critica della ragion pura, Bari 1924, p. 39). He then subdivides a pr. cognitions into pure and non-pure, according as something empirical is or is not mixed with them. The distinctive characteristics of a pr. cognitions, in no way derivable from experience, are absolute necessity and universality (ibid., pp. 39–40).
The Kantian sense of a pr. has remained, both in its more rigorously technical expression and in a broader and more common usage, 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 a pr. and a post. judgments, arguments, and methods.