Etymology
1. c.1225, "statement in an argument," also "intellectual faculty that adopts actions to ends," from Anglo-Fr.
resoun, O.Fr.
raison, from L.
rationem (nom.
ratio) "reckoning, understanding, motive, cause," from
ratus, pp. of
reri "to reckon, think," from PIE base
*rei- "to reason, count" (cf. O.E.
rædan "to advise; see read). Meaning "sanity" is recorded from, c.1380. The verb (c.1300) is from O.Fr.
raisoner, from L.L.
rationare "to discourse." Originally "to question (someone)," sense of "employ reasoning (with someone)" is from 1847, and that of "to think in a logical manner" is from 1593. Phrase
it stands to reason is from 1632.
Age of Reason "the Enlightenment" is first recorded 1794, as the title of Tom Paine's book.
2. From Middle English resoun, reson, from Anglo-Norman raisun (Old French raison), from Latin ratiō, from ratus, past participle of reor (“reckon”), ultimately from Proto-Indo-European *h₂reh₁- (“to think”), reanalysed root of *h₂er- (“to put together”). Displaced native Middle English reden (found in compounds), from Old English rǣden (“condition, stipulation, calculation, direction”), from the same Proto-Indo-European source (compare West Frisian reden (“reason”), Dutch reden (“reason”)). Doublet of ration and ratio. See: Latin '
ratiō', Dutch '
reden'.