NOUMENON. — From the Greek νοούμενον (νοῦς, intellect) it etymologically signifies "intelligible."
The term is used by Plato (v.) to refer to the species or ideas not perceptible by us through the senses, but only by the intellect (ἀνασθητήσας ὑφήμων εἰς δὴ νοούμενος μόνον, Tim., 51 d., cf. Resp., 508). idea (v.) was assumed by Plato to be the true and subsistent reality of which sensible appearances are imitation and participation (v.). Kant, referring to the Platonic meaning, adopted the term n. to denote the "thing or reality in itself" that lies beyond the objects of the senses as phenomena: "I can say that the representation of such beings, which are the ground of phenomena, and thus of pure intelligible beings, is not only admissible but also indispensable" (Prolegomena, part 2, § 32; Italian trans. by P. Carabellese, Messina 1942, pp. 126-27; cf. Critique of Pure Reason, Transcendental Logic, ch. 3, Italian trans., I, Bari 1910, pp. 248, 250). For Kant, such a "reality in itself" is unknowable to human intellect, which relates only, through intuitions and concepts, to sensible objects. He nevertheless distinguishes n. in a negative sense ("a thing insofar as it is not an object of our sensible intuition") and n. in a positive sense ("the object of a non-sensible, i.e., intellectual intuition, which, however, is not ours and whose possibility we cannot even comprehend," ibid., p. 249): n. is therefore, for theoretical human knowledge that cannot go beyond phenomena, "only a limiting concept (Grenzbegriff)" (ibid., p. 251). The term has remained in philosophical language, indicating, with predominantly Kantian reference, real being, reality, the "thing in itself" independent of knowledge.