principle is a fundamental truth, law, or assumption that other reasoning or conduct rests on — a rule general enough that particular cases are derived from it rather than the reverse.

The word covers several related senses. As a rule of conduct it names a standard a person holds in advance of any specific situation, which is what is meant by acting on principle or being a person of principle; the plural usually refers to a whole code rather than a list of separate rules. As a law of nature or of a discipline it names a regularity that explains and predicts — Archimedes' principle, the uncertainty principle, the principle on which an engine works. An older sense, now largely obsolete outside technical usage, is simply origin or source; a related survival is the chemist's use of principle for the active constituent of a substance.

The word entered English in the late fourteenth century from Anglo-French, alongside Old French principe, from Latin principium, "a beginning, commencement, origin," itself from princeps, "first man, chief," a compound of primus ("first") and the root of capere ("to take"). The sense of a foundational tenet on which others rest is recorded by the 1530s, and the scientific application by the early nineteenth century.

The persistent confusion is with principal, which shares the same Latin ancestor but not the same job: principal is chiefly an adjective meaning main or chief, and a noun for a person in authority or a sum of money lent, while principle is only ever a noun meaning a rule or truth. The common mnemonic — a principal is a person or an amount, a principle is a rule — holds in every ordinary case. Reasoning from 📝First Principles applies the word in its strictest reading: stripping an argument back to premises that cannot be derived from anything more basic, then building forward rather than reasoning by analogy.

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