truth, truthfulness
One of the key concepts of Platonic ontology and epistemology: ἀλήθεια denotes both the correspondence of speech to what is — a true or false λόγος (Crat. 385b) — and the very unconcealment of genuine being (etymologically ἀ-λήθεια), that which “truly is” (ὄντως ὄν). True truth is disclosed only in the ascent to the intelligible (Resp. VII 515c); the philosopher’s soul, akin to being, “lays hold of truth” (Resp. VI 490a), though the body and the passions of the senses hinder its attainment (Phaed. 65a).
The direct opposite of truth is falsehood (ψεῦδος): Plato distinguishes a verbal lie from the “true falsehood” (ἀληθῶς ψεῦδος) — the soul’s own error about what is (Resp. II 382a). In cognition, truth is the content of knowledge (ἐπιστήμη) directed at being, whereas opinion (δόξα), which bears on what is intermediate between being and not-being, may be either true or false.
At the level of being, truth is inseparable from being (τὸ ὄν): only that which is “always the same as itself” (Phaed. 78d) truly “is,” and to this truly existent (ἀληθῶς ὄν) are opposed the appearances and images (φάντασμα, εἴδωλον) produced by the art of deception (Soph. 240a).
The source of truth is the good (ἀγαθόν): it “grants truth to the things known and power to the knower” (Resp. VI 508e), and in the Philebus truth enters the triad μέτρον — συμμετρία — ἀλήθεια (measure, proportion, truth), through which the good manifests itself in the mixed life (Phil. 64e).