ignorance of the law is no excuse
Not knowing a rule existed will not get you out of having broken it.
“He said he didn't know the rule, but ignorance of the law is no excuse.”
“The tribunal reminded the company that ignorance of the law is no excuse, particularly for a business with its own legal department.”
“Ignorance of the law is no excuse is a fair rule when the law can be read in an afternoon, and a harder one to defend across a regulatory code that runs to several thousand pages and changes quarterly.”
The principle is set out in the Digest, the sixth-century compilation of classical Roman jurists: ignorance of fact excuses, ignorance of law does not. A man who did not know the horse was stolen is in a different position from a man who did not know that theft was a crime.
Roman law did make allowances — for soldiers, for minors, for women in some periods — but the rule itself was not softened, and for a good structural reason. If not knowing the law were a defence, the law would reward everyone who took care not to find out.
Every legal system descended from Rome kept it, usually in the Latin: ignorantia legis neminem excusat. English states it in English, and it is one of the few legal maxims most people can quote — normally at the moment they have discovered it applies to them.
Roman legal Latin that never left the courtroom — or the boardroom.
- 01
bona fide (idiom)
Genuine and honestly meant — the real thing, not a pretence.
The Roman standard of good faith.
- 02
caveat emptor (idiom)
Let the buyer beware — checking what you are buying is your own problem.
What Rome did not actually say.
- 03
quid pro quo (idiom)
Something given in return for something else, usually as an understood trade.
From the apothecary's substitute.
- 04
de facto (idiom)
In practice, whatever the official position says.
The question of fact.
- 05
de jure (idiom)
According to the law, whether or not that is what actually happens.
The question of law.
- 06
ipso facto (idiom)
By that very fact — the consequence follows automatically, with no further step.
Consequences that need no hearing.
Last updated August 3, 2026