falsehood
In Plato, ψεῦδος is not merely verbal untruth but an ontologically charged category: to speak or to believe falsely is to present "what is not" as "what is", or the reverse (Soph. 240e–241a). In the Republic, Socrates distinguishes two kinds of falsehood: "falsehood in the most important matter and about what is most important" — the soul's (ψυχή) error concerning being (τὸ ὄν), which he calls "true falsehood" (ἀληθῶς ψεῦδος) — and "falsehood in words" (ἐν τοῖς λόγοις), a verbal imitation of the first, sometimes admissible as a medicine (φάρμακον) (Resp. II 382a–c). ψεῦδος is thus first of all a distorted relation of the soul to being, and only secondarily a speech act.
Within the circle of notions bound up with λόγος, ψεῦδος is the direct opposite of truth (ἀλήθεια): where true speech says of being what it is, false speech presents the non-existent as existent. This is possible at all because the forms (εἴδη) interweave with one another and allow the saying of "other about other": false speech (ψευδὴς λόγος) ascribes to its object what does not belong to it (Soph. 263b). Within the soul the same λόγος appears as false belief (ψευδὴς δόξα) — an inner speech whose affirmation or denial does not correspond to what is. The ethical side completes the picture: the genuine philosopher "in every way hates ψεῦδος and loves ἀλήθεια", since the striving for truth is the very order of his soul (Resp. VI 485c).