Ignorantia juris non excusat or Ignorantia legis neminem excusat

(Latin for, "Ignorance of the law does not excuse," or, "Ignorance of the law excuses no one.")

Ignorance of the law excuses no man; not that all men know the law; but because 'tis an excuse every man will plead, and no man can tell how to confute him.
John Selden (1584-1654), posthumously published in Table Talk, 1689.