It is the rule, that everyone is prejudiced by his ignorance of law, but is not prejudiced by his ignorance of a material fact.

Explanation

Ignorance of the law is no excuse, but ignorance of a material fact may be — the law presumes every man knows his legal obligations, while excusing honest ignorance of circumstances.

Where this applies

• A driver is ticketed for going 45 in a 30-mph zone and claims he did not know the speed limit — his ignorance of the traffic law is no defense, because every person is presumed to know the law.
• A homeowner unknowingly buys a house with a hidden structural defect that the seller concealed — his ignorance of the material fact (the defect) may excuse him and give him a claim against the seller, because ignorance of fact can be a defense.
• A taxpayer fails to file a return for a new tax enacted last year, claiming he never heard about it — his ignorance of the tax law harms him because the law presumes he knows his obligations, but if the IRS sent his refund to a wrong address he never knew about, that factual ignorance would not count against him.