POSTULATE. - From Lat. postulare (to require; Gk. αἰτησίω), it is a proposition (v.) that is admitted as true without demonstration (v.) and whose affirmation is necessary for the demonstration of other truths.
According to Aristotle (Anal. post., I, ch. 10, 76 b), a p. is that proposition which the demonstrator asks the learner to admit, when the learner either has no opinion on the matter or even holds an opinion contrary to the p. itself. Hence the definition of p. as τὸ ὑπενεργῶ τοῦ μνηθοσύνης τῇ 80% Axiom (v.), on the other hand, is a proposition whose truth everyone admits. In Euclid's Elements, p. are likewise distinguished from "common notions" (Euclid's common notions are equivalent to Aristotle's axioms), because p. "provide certain elementary constructions from which results the existence of figures satisfying certain conditions" (F. Enriquez, s. v. in Enc. Ital., XXVIII, pp. 99-100; cf. Brunschvicg, v. bibl., p. 91).
Kant speaks of p. in two different senses. In the Critique of Pure Reason, certain principles of the understanding are called "p. of empirical thought," namely those that apply the categories of modality, and they state that: "1) What corresponds to the formal conditions of experience... is possible. 2) What is connected with the material conditions of knowledge is real. 3) What is connected with the real according to universal conditions of experience is necessary" (Kritik d. v. Vernunft, ed. of the Prussian Academy, Berlin 1902, A. 218, B. 265-66). Further on (A. 232-35, B. 285-87) Kant explains that he has not here taken the term p. in the sense of a proposition admitted without justification, but in the sense of a proposition that assigns a character to the object considered only in the way it presents itself to the cognitive faculty. And this, he adds, is the sense that the term p. has in mathematics, where it indicates a proposition with which we give ourselves an object and construct it (ovrengen [A. 234, B. 287]). In the Critique of Practical Reason, the affirmations necessarily connected with the moral law are called p. These are: freedom, the existence of God and the immortality of the soul (ibid., p. 132). While the p. of mathematics have apodictic certainty, and postulate "the possibility of an action whose object has been recognized as possible a priori, theoretically with absolute certainty," that is, they construct an object according to the laws of the pure understanding, the p. of practical reason "postulate the possibility of an object (God and immortality) on the basis of apodictic practical laws" (ibid., p. 11, note); that is, they affirm certain realities as necessary conditions for moral activity - which is commanded categorically by reason - to have a meaning.
Bibliography
Sofia Vanni Rovighi