Logos (UK: /ˈloʊɡɒs, ˈlɒɡɒs/, US: /ˈloʊɡoʊs/; Ancient Greek: λόγος, romanized: lógos, lit. ‘word, discourse, or reason’) is a term used in Western philosophy, psychology and rhetoric, as well as religion (notably Christianity); among its connotations is that of a rational form of discourse that relies on inductive and deductive reasoning.

Aristotle first systematized the usage of the word, making it one of the three principles of rhetoric alongside ethos and pathos. This original use identifies the word closely to the structure and content of language or text. Both Plato and Aristotle used the term logos (along with rhema) to refer to sentences and propositions.

Ancient Greek: λόγος, romanized: lógos, lit. ‘word, discourse, or reason’ is related to Ancient Greek: λέγω, romanized: légō, lit. ‘I say’ which is cognate with Latin: lex, lit. ‘law’. The word derives from a Proto-Indo-European root, *leǵ-, which can have the meanings “I put in order, arrange, gather, choose, count, reckon, discern, say, speak”. In modern usage, it typically connotes the verbs “account”, “measure”, “reason” or “discourse”.^[1][2] It is occasionally used in other contexts, such as for “ratio” in mathematics.^[3]

Logos became a technical term in Western philosophy beginning with Heraclitus (c. 535 – c.  475 BC), who used the term for a principle of order and knowledge.^[4] Ancient Greek philosophers used the term in different ways. The sophists used the term to mean “discourse“. Aristotle applied the term to refer to “reasoned discourse”^[5] or “the argument” in the field of rhetoric, and considered it one of the three modes of persuasion alongside ethos and pathos.^[6] Pyrrhonist philosophers used the term to refer to dogmatic accounts of non-evident matters. The Stoics spoke of the logos spermatikos (the generative principle of the Universe) which foreshadows related concepts in Neoplatonism.^[7]

Within Hellenistic Judaism, Philo (c. 20 BC – c.  50 AD) integrated the term into Jewish philosophy.^[8] Philo distinguished between logos prophorikos (“the utterer word or speaker”), logos spermatikos(“the speech”) and the logos endiathetos (“the word remaining within”).^[9]