word, argument, reason
λόγος is one of the broadest words in Greek philosophy: speech, reasoning, account, definition, ratio — that through which thought acquires articulate form. Plato retains all these shades and binds them into a single work: the articulation of what is in speech. Already in the Cratylus λόγος is defined as that which is true or false (Crat. 385b); in the Sophist it is unfolded as the interweaving of name and verb (ὄνομα and ῥῆμα), necessarily about something and necessarily true or false (Soph. 261e–263b). In the Republic λόγος is bound up with the very essence of a thing: the dialectician is the one "who takes the λόγος of the essence (οὐσία) of each thing" (Resp. VII 534b).
λόγος is the threshold dividing opinion (δόξα) from knowledge (ἐπιστήμη). In the Meno true opinion becomes knowledge only when it is bound by "reasoning out of the cause" (αἰτίας λογισμός), which alone converts it into knowledge (Men. 97e–98a). In the Phaedo Socrates describes his "second sailing": to seek truth not in what is sensible but in arguments themselves (διὰ τῶν λόγων), positing each time the "strongest" λόγος and holding to what agrees with it (Phd. 99e–100a). The Theaetetus tests the definition of knowledge as "true opinion with an account" (μετὰ λόγου): running through three senses of λόγος — vocalization of thought (Tht. 206d), enumeration of elements (Tht. 207a), statement of a distinguishing mark (Tht. 208c) — the dialogue rejects each in turn (Tht. 201c–210a). Thus the very demand "to give an account" (διδόναι λόγον) appears in Plato as the form of knowledge, though its strict accomplishment is reserved for dialectic (διαλεκτική).
In Plato's world λόγος is rooted in the soul (ψυχή): thinking (διάνοια) is "the silent speech that the soul carries on with itself," and opinion (δόξα) is its completion, an affirmation or denial (Soph. 263e–264b; Tht. 189e–190a). The truly "living" λόγος Plato distinguishes from the dead letter: it is the λόγος "written with knowledge in the soul of the learner" (μανθάνων), able to defend itself and knowing before whom to speak and before whom to keep silent (Phaedr. 276a). The supporting connections of logos are truth (ἀλήθεια), knowledge (ἐπιστήμη), opinion (δόξα), thinking (διάνοια), dialectic (διαλεκτική), intellect (νοῦς), soul (ψυχή), name (ὄνομα), verb (ῥῆμα); its opposites are falsehood (ψεῦδος) and the "irrational" (ἄλογον) — that to which λόγος cannot be given.