How to say "theory" in Japanese

• 説せつJLPT N3approx.Common
  noun, noun (suffix)Covers a theory in the sense of a doctrine or a proposed explanation of something, often one put forward by a particular person or school. It is not the word for theory as a systematic body of knowledge.
  あなたがなんと言いおうと、私はやはりその説せつが正ただしいと思おもう。↗

  For all you say, I still believe in the truth of the theory.
• 理論りろんJLPT N1approx.Common
  nounThe word for theory as a systematic body of principles and reasoning, as in the sciences or economics, and the usual counterpart to practice. This is the default choice for most academic and scientific senses of 'theory'.
  その考かんがえが彼かれの理論りろんの基礎きそとなっている。↗

  The idea underlies his theory.
• 理屈りくつJLPT N1approx.Common
  nounCovers theory in the sense of reasoning or logic, but often carries a nuance of quibbling or strained, self-serving argument. It is seldom the word you want for a scientific or academic theory.
  彼かれはその理屈りくつがわからないほどばかではない。↗

  He is too wise not to see the reason.
• セオリーCommon
  nounA loanword used in everyday speech, often for a practical rule of thumb or the accepted way of doing something rather than a formal theory. It suits casual talk about how things are supposedly done.
• 原理げんりJLPT N2approx.Common
  nounGlossed 'theory', but it really points to a fundamental principle or underlying truth on which something works. Choose it when English would naturally say 'principle' rather than 'theory'.
  誰だれもがその考かんがえを原理げんり的てきには認みとめるだろう。↗

  Everyone will accept that idea in principle.

Use 理論 for a systematic scientific or academic theory, 説 for a proposed explanation or doctrine, 原理 for an underlying principle, 「セオリー」 for informal practical rules of thumb, and 理屈 for everyday reasoning, often with a critical nuance.

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

• 説せつtheory
• 理論りろんtheory
• 理屈りくつtheory
• セオリーtheory
• 原理げんりprinciple
• 学説がくせつtheory
• 命題めいだいproposition
• 原論げんろんtheory
• 論ろんtheory (e.g. of evolution)