English jurist John Seldon (1584-1654) is quoted as reflecting, in a 1689 book on his sayings called Table-talk:
"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 refute him."
Best known as a component of the maxim ignorance of the law is no excuse.
The maxim is of ancient origin; the Roman law knew it as ignorantia juris non excusat and it became firmly entrenched in the common law as regards criminal law.
Sometimes codified as in the 2009 Criminal Code of Canada:
"Ignorance of the law by a person who commits an offence is not an excuse for committing that offence."
In Mens Rea in Statutory Offences, jurist Edwards noted that:
"At least one thing is crystal clear, namely, that it does not involve proof that the accused knew that he was committing a criminal offence.
"It has always been accepted as an axiomatic principle that ignorance of the law is no excuse. Were the position otherwise it is obvious that the legislature's handiwork could be flouted indiscriminately, an offender taking care to insure that he did not make himself cognizant with the law."
In R v Crosswell, when charged with breach of probation, said he "misinterpreted (his) probation order". Justice Pringle replied, in convicting:
"... even if I give Mr. Crosswell the benefit of the doubt that he misunderstood his obligations as he said, his mistake was one of law. Generally, ignorance of the law is no excuse...."
There is two significant distinctions and two exceptions. The first distinction is encapsulated in the Latin expression ignorantia facti excusat, ignorantia juris non excusat: ignorance of fact excuses; ignorance of law does not excuse.
Secondly, as succinctly stated by Justice Muldoon in Rollinson:
"The rule of criminal law, ignorantia juris non excusat ... applies only to criminal law."
Or in Lansdown:
"That maxim of law, ignorantia juris non excusat, was in regard to the public, that ignorance cannot be pleaded in excuse of crimes, but did not hold in civil cases."
As to the two exceptions, from R. v. Crosswell:
"Generally, ignorance of the law is no excuse, although there are some exceptions. An exception (is) ... where the charge was one of willful failure to comply with probation and the breach was an allegation of committing a criminal offence with a separate mens rea. A further exception is recognized to the general rule that ignorance of the law is no excuse, for cases of officially induced error."
No discussion on ignorance of the law would be complete without these memorable words of Justice Abbott in Montriou v Jefferys:
"No attorney is bound to know all the law. God forbid that it should be imagined that an attorney, or a counsel, or even a judge is bound to know all the law...."
- Criminal Code RSC 1985 Chapter C-46, §19
- Duhaime, Lloyd, Legal Definition of Ignorantia Juris Non Excusat
- Duhaime, Lloyd, Legal Definition of Officially Induced Error
- Edwards, J., Mens Rea in Statutory Offences (London: MacMillan, 1955)
- Lansdown v. Lansdown (1730), Mos. 364
- Montriou v Jefferys, 2 Car. & P. 113; also at 172 E.R. 51 (1825)
- Rollinson v. Canada 1991 FCJ 25; and also partly at (1991) 3 FC 70
- R v Crosswell 2007 ONCJ 25; also at 2007 OJ 239
- R. v MacDougall  2 SCR 605; also at 142 DLR 3d 216, 44 NR 560, JE 82-1202, 54 NSR 2d 562, 1 CCC 3d 65, 31 CR 3d 1, 18 MVR 180 and 1982 CarswellNS 605