The New International Encyclopædia/Mistake
MISTAKE. An erroneous mental conception affecting the will, and hence leading to or preventing some act. The importance of the mistake in legal contemplation is its effect upon the act. In general a mistake of law produces no legal effect upon an act influenced or induced by it. Thus one is without remedy who has entered into a contract without knowing or understanding the legal effect, or who has voluntarily, and with full knowledge of the facts, paid a claim not well founded in law. The full acceptance of the doctrine is due to a misapplication of the measure Ignorantia juris non excusat (“ignorance of the law does not excuse”), which is applicable only in the criminal law or under statutes imposing quasi-criminal penalties where sound policy requires that ignorance of law should not excuse one charged with a crime. (See Intent.) Mistake of fact has a direct legal effect on many acts, and the law in many cases affords a remedy to one who has done a prejudicial act induced by mistake of fact.
The effect of mutual mistake as to a material term of a contract is to prevent the meeting of the minds, and thus prevent the formation of a contract (q.v.). The effect of mutual mistake of fact in case of sale is to prevent passing of title, and one who has given up the possession of personal property under mistake, preventing the passing of title, may recover the specific property by an appropriate action, or its value in an action of trover. In general, whenever money or property is delivered to another under material mistake of fact, its value may be recovered in an action based on the theory of quasi-contract (q.v.). In the law of tort, the effect of mistake of fact varies considerably with the different branches of the subject. In the law of trespass, one is required to know his own, and he interferes with the property of another at his peril; hence mistake of fact does not affect his liability. The same is true of libel and slander. Equity has jurisdiction to relieve one from the consequences of his mistake of fact, by compelling a reformation or rescission of a contract.