Translation guide
The English word "theory" covers a range of meanings from a scientific explanation to a personal guess. In Japanese, different words are used depending on whether you mean a formal academic theory, a hypothesis, or a casual speculation.
A well-substantiated explanation of some aspect of the natural world, based on a body of facts and repeatedly confirmed through observation and experiment.
The standard term for a systematic, academic, or scientific theory. Used in fields like physics, economics, music, etc.
アインシュタインの相対性理論は有名です。
Einstein's theory of relativity is famous.
彼は経済理論を研究している。
He is researching economic theory.
Refers to a theory or doctrine within a specific academic field, often implying a school of thought.
その学説は多くの学者に支持されている。
That theory is supported by many scholars.
A proposed explanation for something, still needing verification; a supposition.
A hypothesis or tentative theory, often used in scientific contexts but also for any reasoned guess.
その仮説はまだ証明されていない。
That hypothesis hasn't been proven yet.
彼は新しい仮説を立てた。
He formulated a new hypothesis.
Inference or reasoning; a conclusion drawn from evidence, often used in logic or detective work.
An idea used to account for a situation or justify a course of action, often without solid evidence; a personal opinion.
Speculation, conjecture, guesswork. Often used when someone is making assumptions without proof.
それはただの憶測だ。
That's just a theory (speculation).
憶測でものを言ってはいけない。
Don't talk based on speculation.
Guess, conjecture, surmise. Similar to 憶測 but slightly more neutral; can be based on some reasoning.
Literally "the theory that ~". A flexible way to present a personal or unverified idea.
彼が犯人だという説もある。
There's also a theory that he's the culprit.
The general principles or ideas of a subject, as opposed to its practical application.
Used in the phrase 理論上 (theoretically) or contrasted with 実践 (practice).
理論上は可能だが、実際には難しい。
In theory it's possible, but in practice it's difficult.
理論と実践は違う。
Theory and practice are different.
理論 (riron) is a systematic theory, like in science. 仮説 (kasetsu) is a hypothesis, a proposed explanation awaiting verification. 憶測 (okusoku) is mere speculation or guesswork, often with a negative nuance of being unfounded. Choose based on how much evidence and rigor is involved.
進化論は理論であり、仮説ではない。
Evolution is a theory, not a hypothesis.
彼の話は憶測に過ぎない。
His story is nothing but speculation.
The loanword セオリー (seorii) exists but is mainly used in specific contexts like game strategy or sports tactics. It does not cover the full range of English "theory" and can sound unnatural if used for scientific theories or personal guesses.
理論上はうまくいくはずだ。
In theory, it should work.
それは単なる推論に過ぎない。
That's just a theory (inference).
私の推測では、彼は来ないだろう。
My theory is that he won't come.