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]