rea·son / ˈrēzən/ • n. 1. a cause, explanation, or justification for an action or event: the minister resigned for personal reasons it is hard to know for the simple reason that few records survive. ∎  good or obvious cause to do something: we have reason to celebrate. ∎  Logic a premise of an argument in support of a belief, esp. a minor premise when given after the conclusion.2. the power of the mind to think, understand, and form judgments by a process of logic: there is a close connection between reason and emotion. ∎  what is right, practical, or possible; common sense: people are willing, within reason, to pay for schooling. ∎  (one's reason) one's sanity: she is in danger of losing her reason.• v. [intr.] think, understand, and form judgments by a process of logic: humans do not reason entirely from facts | [as n.] (reasoning) the present chapter will outline the reasoning behind the review. ∎  [tr.] (reason something out) find an answer to a problem by considering various possible solutions. ∎  (reason with) persuade (someone) with rational argument: I tried to reason with her, but without success.PHRASES: beyond (all) reason to a foolishly excessive degree: he indulged Andrew beyond all reason.by reason of formal because of: persons who, by reason of age, are in need of care.for some reason used to convey that one doesn't know the reason for a particular state of affairs, often with the implication that one finds it strange or surprising: for some reason he likes you.listen to reason be persuaded to act sensibly: the child is usually too emotionally overwrought to listen to reason.theirs (or ours) not to reason why used to suggest that it is not someone's (or someone else's) place to question a situation or system.reason of state another term for raison d'état. (it) stands to reason it is obvious or logical: it stands to reason that if you can eradicate the fear, the nervousness will subside.DERIVATIVES: rea·son·er / ˈrēz(ə)nər/ n.rea·son·less adj. ( archaic ).ORIGIN: Middle English: from Old French reisun (noun), raisoner (verb), from a variant of Latin ratio(n-), from the verb reri ‘consider.’