Speech in the soul (λόγος ἐν τῇ ψυχῇ) is the formula by which Plato describes thinking as the soul’s silent conversation with itself. In the Theaetetus (189e) thinking (διάνοια) is defined as «the speech that the soul goes through with itself about whatever it considers» (λόγον ὃν αὐτὴ πρὸς αὑτὴν ἡ ψυχὴ διεξέρχεται περὶ ὧν ἂν σκοπῇ); in the Sophist (263e) this same inner λόγος — «the soundless inner dialogue of the soul with itself» (ὁ ἐντὸς τῆς ψυχῆς πρὸς αὑτὴν διάλογος ἄνευ φωνῆς) — is opposed to spoken speech: «the stream that goes out through the mouth with sound» (ῥεῦμα διὰ τοῦ στόματος ἰὸν μετὰ φθόγγου), which is what is called λόγος.
When this inner λόγος ends in a silent affirmation or denial, it becomes opinion (δόξα) (Soph. 264a; cf. Theaetetus 190a). The formula pairs with the «dialogue of the soul» (διάλογος τῆς ψυχῆς) and binds λόγος to the soul (ψυχή): the account is carried on «within» before it is clothed in audible speech. Thus δόξα turns out to be the outcome of speech in the soul, not a faculty separate from λόγος.