Word Origin & History
early 13c., "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, late 14c. 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 1590s. Phrase it stands to reason is from 1630s. Age of Reason "the Enlightenment" is first recorded 1794, as the title of Tom Paine's book.