Logos is one of the most capacious words in Greek philosophy, with a rich polysemy: "reason," "word," "speech," "ratio," "principle," "formula." In the Stoic system the Logos occupies a central place — it is the single explanatory principle through which physics, theology, psychology, and ethics are all described.

The Stoic Logos has several levels:

• The cosmic Logos — the creative reason, immanent in the cosmos, identical with god, Zeus, fate, providence, and the nature of the whole (TERMτοῦ ὅλου φύσις). It is "the artificing fire" (πῦρ τεχνικόν) that pervades all things as TERMpneuma and secures their coherence and purposiveness.
• The seminal logoi (λόγοι σπερματικοί) — "seeds" of the cosmic reason that unfold in particular things as their essential constitution. Every thing unfolds out of its own logos spermatikos.
• The individual logos of the human being — the rational part of the soul, identical with the TERMruling part, the TERMintellect, the "god within us" (δαίμων), and the TERMdivine portion. It is the cosmic Logos as locally manifested — identical in nature with the whole.

The Stoics also distinguish:

• λόγος ἐνδιάθετος — "inner logos," mental content;
• λόγος προφορικός — "uttered logos," articulated speech.

This distinction allows them to speak of a kind of rationality in animals (they have something like endiathetos but no prophorikos) and grounds the unique status of the human being as the only fully rational creature in the cosmos.

SVF I 85, 87 (Zeno on λόγος as τεχνικῷ πυρί, the artificing fire); II 1009 (the cosmic Logos); II 458, 632 (λόγος as pneuma); III 4–9 (the role of the Logos in Stoic ethics — to live κατὰ λόγον); DL VII 134–140 (the doctrine of the Logos and the δύο ἀρχαί, two first principles); Cic. De nat. deor. II 19, 47–58 (Balbus' exposition); LS 44 ("The Stoic universe"), 53 ("Soul"). In Marcus the Logos is a leitmotif: Med. 2.1; 3.16; 4.4; 4.13 ("the common λόγος"); 4.40; 5.27; 7.9 ("a sacred bond"); 12.5; 12.30 ("one light of the sun … one intellectual element").

The Stoic Logos is not "god" in the Abrahamic sense (transcendent, personal, separate from the cosmos) but an immanent rational principle, identical with the material cosmos. The Stoics are materialists: the Logos is a body (σῶμα), a particular form of subtle fiery pneuma. This makes their theology specific — the divine is not separated from the world but pervades it as its own reason.

The influence is enormous: through Philo of Alexandria the Stoic Logos enters early Christian theology (John 1:1: «Ἐν ἀρχῇ ἦν ὁ Λόγος» — "In the beginning was the Logos"). In John the Logos has already been theisticised — but the semantic bridge from Stoic ontology is plain.

In Marcus the Logos plays the role of a unifying category: to live according to the Logos = to live κατὰ φύσιν = to live as part of the one cosmos = to realise one's own TERMinner reason. See the famous formula at Med. 4.4: «εἰ μὲν τὸ νοερὸν ἡμῖν κοινόν, καὶ ὁ λόγος... κοινός...» — "if the intellectual element is common to us, then so too is reason."