reason early 13c., "statement in an argument," also "intellectual faculty that adopts actions to ends," from Anglo-Fr. resoun, . 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. rædan "to advise; see read). Meaning "sanity" is recorded from, late 14c. The verb () is from . raisoner, from . 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.