mind, purpose, sense
Νοῦς in Plato is the highest cognitive faculty of the soul (ψυχή), the organ of immediate vision of the Forms and above all of the Good (ἀγαθόν). In the Republic it is placed on the upper segment of the "divided line": where knowledge no longer moves through hypotheses and images, but "touches" (ἅπτεται) the intelligible itself (τὸ νοητόν), and this highest act is called νόησις (Resp. VI 511b–e). In the sun analogy the Good "gives truth to what is known and the power [to know] to the knower" (Resp. VI 508e): νοῦς is to the intelligible what sight is to the visible, and the Good is for it the light and the cause of truth (ἀλήθεια). Νοῦς is thus the capacity of the soul to stand face to face with being (τὸ ὄν) and to receive from it its own measure.
Unlike sense-perception (αἴσθησις) and opinion (δόξα), which slide over what becomes, νοῦς holds steadily upon being, and therefore it alone bears genuine knowledge (ἐπιστήμη) of the Forms. In Plato νοῦς also has a cosmological dimension: in the Timaeus "there is nothing finer than νοῦς in soul and nothing finer than soul in body" (Tim. 30b); in the Philebus he is called the one who "rules over heaven and earth" (Phlb. 28c); and in the Sophist, without νοῦς and soul, "the whole of being" would not be alive (Soph. 248e–249a). Within the soul νοῦς is its highest part and the root both of discursive thinking (διάνοια) and of immediate intellection (νόησις); through λόγος, which articulates the Forms, it is bound to truth and stands against ψεῦδος and ἄγνοια.