Maxims, axioms & idioms · a browsable collection

Legal maxims, political axioms, and the idioms that carry them into everyday argument — each with a plain-English gloss and where it comes from. We state what a maxim means and where it's from; we do not tell you it is right. Open any maxim to see the case law that cites or applies it — curated landmark cases plus a live search of the public record. The legal maxims cross-reference the same categories as our case law.

Legal maxims Political axioms Idioms & mottos All

Ignorantia juris non excusat legal · Latin

“Ignorance of the law excuses not”

You are bound by the law whether or not you know it exists.

Provenance: Roman-law maxim; standard in common-law systems.

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Actus reus non facit reum nisi mens sit rea legal · Latin

“An act does not make one guilty unless the mind is also guilty”

Most crimes require both a wrongful act and a guilty mental state.

Provenance: Coke; foundational to Anglo-American criminal law.

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Nemo judex in causa sua legal · Latin

“No one should be a judge in his own cause”

A decision-maker must not have a personal stake in the matter — the rule against bias.

Provenance: Natural-justice maxim; Dr. Bonham's Case (1610).

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Audi alteram partem legal · Latin

“Hear the other side”

No one should be condemned unheard — the core of due process.

Provenance: Roman/natural-justice maxim.

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Stare decisis legal · Latin

“To stand by things decided”

Courts follow prior rulings — the engine of case law.

Provenance: Common-law doctrine of precedent.

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Respondeat superior legal · Latin

“Let the superior answer”

An employer is liable for torts an employee commits within the scope of employment (vicarious liability).

Provenance: Common-law agency doctrine.

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Nulla poena sine lege legal · Latin

“No penalty without a law”

You can only be punished under a law that existed when you acted (anti-ex-post-facto).

Provenance: Enlightenment penal principle; Feuerbach.

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Ei incumbit probatio qui dicit, non qui negat legal · Latin

“The burden of proof is on the one who asserts, not the one who denies”

The party making a claim must prove it.

Provenance: Roman law (Digest).

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Res ipsa loquitur legal · Latin

“The thing speaks for itself”

Some accidents so obviously imply negligence that the fact of them is evidence.

Provenance: Byrne v. Boadle (1863).

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De minimis non curat lex legal · Latin

“The law does not concern itself with trifles”

Courts won't act on trivial matters.

Provenance: Common-law maxim.

Cases applying this maxim

Wisconsin Dept. of Revenue v. William Wrigley, Jr., Co. 505 U.S. 214 (1992)

The Court called the maxim 'part of the established background of legal principles against which all enactments are adopted.'

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Habeas corpus legal · Latin

“That you have the body”

A court order requiring a jailer to justify holding a prisoner — the great writ against unlawful detention.

Provenance: English common law; U.S. Const. art. I, §9.

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Cui bono legal · Latin

“To whose benefit?”

Ask who profits — a guide to motive.

Provenance: Cicero, quoting Cassius.

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Fiat justitia ruat caelum legal · Latin

“Let justice be done though the heavens fall”

Justice must be done regardless of consequence.

Provenance: Roman maxim; Somerset v Stewart (1772).

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