POSTULATE. — From Lat. postulare (to request; Gr. αἰτησίω), proposition (v.) that demonstration (v.) and whose affirmation is necessary for the demonstration of other truths.
According to Aristotle (Anal. post., I, chap. 10, 76 b), p. is the proposition which the demonstrator asks the learner to admit, when the learner either has no opinion at all on the matter or actually has an opinion contrary to 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 correspond to Aristotle’s axioms), because p. “provide certain elementary constructions from which follows the existence of figures satisfying certain conditions” (F. Enriquez, s. V. ENOCH. Ital., XXVIII, pp. 99–100; cf. Brunschvicg, V. BIBLICA., 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 state that: “1) That which agrees with the formal conditions of experience … is possible. 2) That which is connected with the material conditions of knowledge is actual. 3) That which is connected with the actual in accordance with universal conditions of experience is necessary” (Kritik d. V. Vernunft, ed. of the Prussian Academy, Berlin 1902, A. 218, B. 265–66). Later (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 in which 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 by which we give ourselves an object and construct it (ovrengen [A. 234, B. 287]). In the Critique of Practical Reason, assertions necessarily connected with the moral law are called p. They are: freedom, the existence of God, and the immortality of the soul (ibid., p. 132). Whereas 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, 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 categorically commanded by reason—to have a meaning.
Sofia Vanni Rovighi