False speech (ψευδὴς λόγος) is a central theme of the Sophist. The Eleatic Stranger calls false a λόγος that «says of its subject things other than are the case» (λέγει ἕτερα τῶν ὄντων): with the example «Theaetetus flies» he shows that the utterance keeps the grammatical bond of noun (ὄνομα) and verb (ῥῆμα) yet ascribes to its subject what is not in fact so (Soph. 263b, 263d).
Plato derives the possibility of false speech from a reinterpreted not-being (μὴ ὄν): to «say what is not» one must grant that not-being in a way is (Soph. 260c). False speech must be distinguished from falsehood in general (ψεῦδος) and from false opinion (ψευδὴς δόξα) — the same false speech completed «in the soul» by a silent affirmation (Soph. 264b); cf. the «true lie» (ἀληθῶς ψεῦδος), deception in the soul itself (Resp. II 382b).