§ 147 Mistake of Fact
In respect of the influence of ignorance or error upon legal liability we have inherited from Roman law a familiar distinction between law and fact. By reason of his ignorance of the law no man will be excused, but it is commonly said that inevitable ignorance of fact is a good defence.383 This, however, is far from an accurate statement of English law. It is much more nearly correct to say that mistake of fact is an excuse only within the sphere of the criminal law, while in the civil law responsibility is commonly absolute in this respect. So far as civil liability is concerned, it is a general principle of our law that he who intentionally interferes with the person, property, reputation, or other rightful interests of another does so at his peril, and will not be heard to allege that he believed in good faith and on reasonable grounds in the existence of some circumstance which justified his act. If I trespass upon another man’s land, it is no defence to me that I believed it on good grounds to be my own. If in absolute innocence and under an inevitable mistake of fact I meddle with another’s goods, I am liable for all loss incurred by the true owner.384 If, intending to arrest A., I arrest B. by mistake instead, I am absolutely liable to him notwithstanding the greatest care taken by me to ascertain his identity. If I falsely but innocently make a defamatory statement about another, I am liable to him however careful I may have been to ascertain the truth. There are, indeed, exceptions to this rule of absolute civil liability for mistake of fact, but they are not of such number or importance as to cast any doubt on the validity of the general principle.
In the criminal law, on the other hand, the matter is otherwise, and it is here that the contrast between mistake of law and mistake of fact finds its true application. Absolute criminal responsibility for a mistake of fact is quite exceptional. An instance of it is the liability of him who abducts a girl under the legal age of consent. Inevitable mistake as to her age is no defence; he must take the risk.385
A word may be said as to the historical origin of this failure of English law to recognise inevitable mistake as a ground of exemption from civil liability. Ancient modes of procedure and proof were not adapted for inquiries into mental conditions. By the practical difficulties of proof early law was driven to attach exclusive importance to overt acts. The subjective elements of wrongdoing were largely beyond proof or knowledge, and were therefore disregarded as far as possible. It was a rule of our law that intent and knowledge were not matters that could be proved or put in issue. “It is common learning,” said one of the judges of King Edward IV., “that the intent of a man will not be tried, for the devil himself knoweth not the intent of a man.”386 The sole question which the courts would entertain was whether the defendant did the act complained of. Whether he did it ignorantly or with guilty knowledge was entirely immaterial. This rule, however, was restricted to civil liability. It was early recognised that criminal responsibility was too serious a thing to be imposed upon an innocent man simply for the sake of avoiding a difficult inquiry into his knowledge and intention. In the case of civil liability, on the other hand, the rule was general. The success with which it has maintained itself in modern law is due in part to its undeniable utility in obviating inconvenient or even impracticable inquiries, and in part to the influence of the conception of redress in minimising the importance of the formal condition of penal liability.