理由|りゆうriyuu[0]heiban (flat)

1. nounreason; grounds; pretext; excuse; motive
   どうしてあなたにお詫わびする理由りゆうがあるのですか。↗

   Why should I apologize to you?

• Synonym訳わけNeutralThe colloquial, everyday word for a reason or cause; it also covers meaning and sense, as in 「訳が分からない」. 「理由」 is more formal and stresses the logical cause.
• Synonym根拠こんきょWrittenA written word for the basis that backs up a claim, as in 「根拠のない噂」; it stresses supporting evidence rather than the motive behind something.
• Synonym由よしWrittenAn archaic, elegant word for a reason or cause; in modern Japanese it survives mostly in 「〜の由」, meaning 'I hear that', as in 「ご病気の由」.
• Synonym事由じゆうWrittenA formal term used in legal and official documents for the reason in question, as in 「退職事由」 and 「解雇事由」.
• Synonym事情じじょうNeutralThe circumstances and background of a matter, as in 「家庭の事情」; it stresses the situation as a whole rather than the reason itself.
• Synonym一理いちりWrittenA measure of truth or a valid point; mostly in 「一理ある」, to agree with part of what someone says.
• Synonym道理どうりNeutralReason or sense that accords with what is right or natural; also in 「道理で」, meaning 'no wonder'.
• Synonym筋合すじあいWrittenGrounds for something being only natural or one's due; mostly in negatives like 「言われる筋合いはない」.
• Synonym所以ゆえんWrittenA written word for why something is as it is, with a philosophical flavour, as in 「人の人たる所以」.
• Synonym原由げんゆWrittenA written word for a cause or reason, but very rare; in everyday speech say 「原因」 or 「理由」 instead.
• Related由来ゆらいWrittenThe origin or derivation of a thing, as in 「地名の由来」; it concerns where something comes from, not the cause of an event.
• Related理屈りくつNeutralLogic or reasoning, but often with a negative shade of quibbling or strained argument, as in 「理屈をこねる」.
• Related裏付けうらづけNeutralBacking or corroborating evidence for a claim, as in 「データによる裏付け」.
• Related証拠しょうこNeutralEvidence proving a fact, as in 「証拠を集める」; covers both physical evidence and testimony.
• Related証明しょうめいNeutralThe act of proving something true; related to 「理由」 but it cannot simply replace it.
• RelatedいわれWrittenThe history or story behind something; also in 「謂れのない非難」, criticism with no grounds at all.
• Related訳柄わけがらWrittenA written word for a reason together with the circumstances of the moment; very rare — say 「事情」 in daily life.
• Related拠り所よりどころWrittenSomething to rely on — grounds to stand on, or an emotional anchor, as in 「心の拠り所」.
• Related成因せいいんWrittenThe cause of how something forms; mainly a scientific term, as in 「台風の成因」.
• RelatedかどWrittenA formal noun used mostly in stating a charge or offence, as in 「窃盗の廉で逮捕」.

Use 「理由」 for the reason you do something or reach a judgement. In casual speech say 「訳」; for the basis behind a claim use 「根拠」; for background circumstances 「事情」; for the objective cause of an event 「原因」; for the origin of a thing 「由来」; for evidence 「証拠」 or 「裏付け」; for logic and sense 「道理」 or 「理屈」; in legal or official writing 「事由」. The rest are rare or literary: 「由」, 「所以」, 「原由」, 「謂れ」, 「訳柄」, 「拠り所」, 「成因」, 「廉」, 「一理」, 「筋合」. 「証明」 means proving something true and is no substitute.

Usage comparisons are AI-assisted and cross-checked by multiple models; for study reference only

1. ある事象じしょうの原因げんいんを論理ろんりづけたもの。