A PRIORI and A POSTERIORI. - Technical expressions of philosophical language with varied meanings. Their origin goes back to Aristotle, who in *Anal. Poster.*, I, 2, 71 b, 33 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, while *priora* and *notiora quoad se* are universals. From this Aristotelian distinction came the common usage of medieval Scholasticism, which gave *a priori* and *a posteriori* an exclusively logical and methodological meaning in relation to a particular kind of reasoning: *a priori* is the argument that proceeds from the general to the particular, from genus to species, from cause to effect, from essence to properties, while *a posteriori* is the argument that takes the opposite path, from the particular to the universal, from effect to cause, from properties to essence. This meaning of *a priori* and *a posteriori*, already present in St. Thomas Aquinas, who denies the possibility of an *a priori* 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, a rather epistemological meaning has been superimposed upon, and has prevailed over, the medieval logical sense, according to which all knowledge that, in some way, does not depend on experience is called *a priori*, and that which derives from experience is called *a posteriori*. This sense is already found in Hume (*Treatise of Human Nature*, I, p. iv) and in Leibniz (*Nouveaux Essais*, IV, 9, 2; Italian trans., Bari 1926, II, p. 194). The meaning of *a priori* became universally central mainly through Kant, in whose system *a priori* knowledge occupies a central position. Kant distinguishes empirical knowledge, derived from experience, and *a priori* knowledge, which, understood in a strict sense, is absolutely independent of all experience, indeed presupposed by experience itself (*Critique of Pure Reason*, Bari 1924, p. 39). He then subdivides *a priori* knowledge into pure and impure, according to whether something empirical is mixed with it or not. The distinctive characteristics of *a priori* knowledge, in no way derivable from experience, are absolute necessity and universality (ibid., pp. 39-40).
Kant’s sense of *a priori* has remained, both in its most rigorously technical expression and in a broader, more common reference, extended to everything that can be known by reason alone, independent of this or that experience; in this sense, the expressions have entered even into common language, which distinguishes *a priori* and *a posteriori* judgments, reasoning, and methods.