Translation guide
The English word "irrational" covers a range of meanings from illogical thinking to unreasonable behavior. In Japanese, different words and phrases are used depending on whether you mean contrary to logic, emotionally driven, or mathematically irrational. This guide helps you choose the most natural expression.
Describing thoughts, arguments, or decisions that lack logical basis.
The most direct and common translation for 'irrational' in the sense of being illogical or not based on reason. Used in both formal and everyday contexts.
彼の主張は不合理だ。
His argument is irrational.
不合理な決定に反対する。
I oppose the irrational decision.
Literally 'non-logical'. Emphasizes a lack of logical structure. Slightly more formal and often used in academic or technical discussions.
その結論は非論理的だ。
That conclusion is irrational.
A common colloquial phrase meaning 'doesn't make sense' or 'unreasonable'. Used in everyday speech to describe something that defies logic.
彼の行動は理屈に合わない。
His behavior is irrational.
Describing actions, demands, or people that are excessive, unfair, or driven by emotion rather than reason.
Conveys a strong sense of unreasonableness and unfairness, often with emotional weight. Commonly used when someone is being unreasonable or making excessive demands.
上司の要求は理不尽だ。
The boss's demands are irrational.
理不尽な理由で怒られた。
I got scolded for an irrational reason.
Means 'absurd' or 'unreasonable', often implying recklessness. Used in casual contexts to describe extreme or irrational actions.
無茶な計画だね。
That's an irrational plan.
Means 'emotional'. When someone is acting irrationally due to emotions, this adjective is often used. It implies a lack of rational control.
彼女は感情的な反応をした。
She had an irrational reaction.
Referring to numbers that cannot be expressed as a simple fraction, like π or √2.
不合理 (fugōri) focuses on logical inconsistency, while 理不尽 (rifujin) emphasizes unfairness and emotional unreasonableness. Use 不合理 for flawed arguments, and 理不尽 for unreasonable demands or treatment.
The phrase 'irrational fear' is often translated as 不合理な恐怖, but a more natural expression is 根拠のない恐怖 (fear without basis) or simply 恐怖症 (phobia) if it's a specific phobia.