Mistake of fact is a controversial defence that conceptually depends on the mens rea of the particular offence. If subjective awareness of prohibited circumstances is required, the Crown will not be able to establish the fault element if the accused honestly, but not necessarily reasonably, believes those circumstances do not exist. On the other hand, if the fault element of the offence requires only that a reasonable person would have recognized the prohibited circumstance or risk, then any defence based on mistake must be honest and reasonable. Finally, even a reasonable mistake of fact would not be a defence to an absolute liability offence. All that would matter would be whether the accused in fact committed the prohibited act.
Deducing the existence and nature of a mistake of fact defence from the fault element of the offence may, however, be too mechanical. It is possible that a mistake of fact may be based on a combination of subjective and objective factors. For example, section 273.2(b) of the Criminal Code now provides that the accused’s belief that the complainant consented to the sexual activity is not a defence if the accused did not take reasonable steps, in the circumstances known to him at the time, to ascertain that the complainant was consenting. As will be discussed below, this creatively combines objective and subjective fault elements.
Mistake of fact defences have frequently been influenced by policy concerns not derived from the fault element of the particular offence. Courts were initially reluctant to accept mistake of fact as a defence. In R. v. Prince,137an accused was convicted of unlawfully abducting an unmarried girl under sixteen years of age without her parents’ permission, even though he had an honest and reasonable belief that the girl was eighteen. Bramwell B. concluded: "The legislature has enacted that if anyone does this wrong act, he does it at the risk of her turning out to be under sixteen."138He did, however, concede that an accused would have no mens rea if he made a subjective mistake about other elements of the offence, such as whether he had received permission from the parents. Only one judge would have acquitted the accused on the basis that
a mistake of facts, on reasonable grounds, to the extent that if the facts were as believed the acts of the prisoner would make him guilty of no criminal offence at all, is an excuse, and that such excuse is implied in every criminal charge and every criminal enactment in England.139The above passage was adopted in the subsequent case of Tolson,140so that a mistake of fact was originally recognized as a defence on the basis that it must be both honest and reasonable.
With the rise of subjective mens rea, courts began to recognize the possibility that an accused could have an honest but not necessarily reasonable belief in a state of circumstances that would make his activity innocent. In R. v. Rees,141the accused was charged with knowingly contributing to the delinquency of a person under eighteen who he believed to be older. Cartwright J. concluded:
[T]he essential question is whether the belief entertained by the accused is an honest one and that the existence or non-existence of reasonable grounds for such a belief is merely relevant evidence to be weighed by the tribunal of fact in determining such essential question.
He conceded that the issue might be different if Parliament had not used the word "knowingly," or had specifically excluded a mistake about the girl’s age as a possible defence. A year later, Cartwright J. acquitted an accused found in possession of an illegal drug because the accused thought that the substance was harmless. He stressed that the offence required subjective mens rea, and that "in a criminal case there is in law no possession without knowledge of the character of the forbidden substance."142Two judges dissented, on the basis that Parliament had intended to enact an absolute prohibition of being in possession of the illegal drugs. An absolute liability offence would, of course, allow no defence of mistake of fact.143An offence based on negligence would af-
ford a defence of mistake of fact "if the accused reasonably believed in a mistaken set of facts which, if true, would render the act or omission innocent."14...