1. a cause, explanation, or justification for an action or event: 行為や出来事の原因、説明、正当化

«事の/…の背後にある» 理由, わけ; 動機 «for, of/behind» (!ofはby reason of A, for reasons of Aなどの成句を除いて｟まれ｠ )

e.g. the minister resigned for personal reasons: その大臣は個人的な理由で辞任した。

e.g. it is hard to know for the simple reason that few records survive.: レコードがほとんど残らなかった単純な理由を知るのは難しい。

good or obvious cause to do something: 何かをする良い、明らかな原因

〖reason (for A) to do/for (A/A's) doing〗 (A〈人〉が)…する理由

e.g. we have reason to celebrate.: 私たちは祝う理由があった。

Logic a premise of an argument in support of a belief, especially a minor premise when given after the conclusion.: 確信を支持する議論の根拠、特に結論の後から与えられる時のさほど大きくない根拠

2. the power of the mind to think, understand, and form judgments by a process of logic: 論理の過程で考える、理解する、判断を作る知性の力

判断力, 思考力, 理性;

e.g. there is a close connection between reason and emotion.: 理性と感情の間には密接なつながりがある。

what is right, practical, or possible; common sense: 正しい、実践的な、可能なもの; 常識

道理, 理屈; 分別

e.g. people are willing, within reason, to pay for schooling.: 人々は道理の範囲内で学校教育に進んでお金を払っている。

(one's reason) one's sanity: 正気

e.g. she is in danger of losing her reason.: 彼女は正気を失う危険にあった。

think, understand, and form judgments by a process of logic: 論理の過程で考える、理解する、判断を作る

｟かたく｠ ; 〖～ that節〗 (論理的に)…と判断する, 推論する; 〖挿入節〗…と推論する (!コーパス(1)伝達部は文中のほか, 文尾も可能; (2) 直接話法の伝達部として文中のほか, 文尾も可能; )

e.g. humans do not reason entirely from facts.: 人間は完全には事実から判断することができない。

with object (reason something out) find an answer to a problem by considering various possible solutions.: 様々な可能な解決法を考慮して問題に対する答えを見つける

(reason with) persuade (someone) with rational argument: (誰かを)理性的な議論で説得する

(論理的に)〈人〉を説得する, 納得させる;

e.g. I tried to reason with her, but without success.: 私は彼女を説得するのを試みたが成功しなかった。

beyond all reason (beyond reason)

to a foolishly excessive degree: 愚かなほど程度を越えるまで

途方もなく, 理屈をこえて

e.g. he indulged Andrew beyond all reason.: 彼は途方もなくアンドリューを甘やかした。

formal because of: ~の理由で

｟かたく｠ A〈事〉の理由により.

e.g. persons who, by reason of age, are in need of care.: 年齢が理由でケアが必要な人

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:

｟主に話｠ どういうわけか, よくはわからないが

e.g. for some reason he likes you.

be persuaded to act sensibly:

e.g. the child is usually too emotionally overwrought to listen to reason.

used to suggest that it is not someone's (or someone else's) place to question a situation or system. with allusion to Tennyson's ‘Charge of the Light Brigade’ (1854).

a purely political reason for action on the part of a ruler or government, especially where a departure from openness, justice, or honesty is involved:

e.g. the king returned that he had reasons of state for all he did.

it is obvious or logical: 明らかだ、論理的だ

｟くだけて｠ «…というのは» 当然である, 道理に合っている «(that)節»

e.g. it stands to reason that if you can eradicate the fear the nervousness will subside.

reasoner | ˈrēz(ə)nər | noun

reasonless | ˈrēz(ə)nləs | adjective

Middle English: from Old French reisun (noun), raisoner (verb), from a variant of Latin ratio(n-), from the verb reri ‘consider’.

The construction the reason why … has been objected to on the grounds that the subordinate clause should express a statement, using a that-clause, not imply a question with a why-clause: the reason (that) I decided not to phone, rather than the reason why I decided not to phone. The reason why has been called a redundancy to be avoided, but it is a mild one, and idiomatic.

An objection is also made to the construction the reason … is because, as in the reason I didn't phone is because my mother has been ill. The objection is made on the grounds that either “because” or “the reason” is redundant; it is better to use the word that instead ( the reason I didn't phone is that …) or rephrase altogether ( I didn't phone because …).

Nevertheless, both the above usages are well established and, although they may be inelegant, they are generally accepted in standard English.