Mistake

Traditionally, exculpatory mistakes have been treated in Anglo-American law as follows. If one engages in the prohibited conduct by mistake (as to what conduct one is engaging in), or causes the harm the law seeks to avoid by mistake (as to the riskiness of one's conduct), then, unless the crime requires the specific intent to engage in the particular conduct or cause the harm—in which case a mistake entails the absence of such an intent and therefore the absence of the crime—one is excused from criminal liability if, and only if, one's mistake is "reasonable." On the other hand, if one makes no mistake about what one is doing, or one's mistake is "unreasonable" (and no specific intent is required), then one is guilty of a criminal violation even if one does not realize the conduct is illegal, and even if that mistake is a reasonable one. In other words, although reasonable mistakes of fact will exculpate, mistakes over the existence or meaning of the criminal law itself, no matter how reasonable, will not. This last point is reflected in the aphorism "Ignorance of the law is no excuse."

The traditional approach is quite confusing and misleading in several respects. First, it treats a claim of mistake as an excuse for having committed a crime rather than as a denial that any crime has occurred. Secondly, and relatedly, it places the burden of proving the mistake on the defendant when it is the prosecution that must prove the crime. Third, and contrary to the popular aphorism, mistakes of law frequently exculpate and cannot be neatly distinguished from mistakes of fact.

To illustrate the first and second confusions in the traditional approach, consider Jane, who fires a fatal bullet at Joe and is charged with criminal homicide. Jane claims that she was hunting and believed Joe was a deer. On the traditional approach, Jane is guilty of homicide unless she proves that she made a reasonable mistake.

But suppose the criminal homicide she is charged with is "intentional homicide," which requires that she kill a human being knowingly or purposefully. Theoretically, the prosecution must prove that she knew or intended to kill a human being. This requirement is completely at odds with the requirement that Jane prove that she made a reasonable mistake regarding what she was shooting. If she made the mistake she claims, then whether or not her mistake was "reasonable," she did not commit the crime of intentional homicide in the first place and should not need to offer a defense to such a crime. Rather, the prosecution should have to prove—beyond a reasonable doubt—that Jane was not mistaken. For it is the prosecution's burden to prove the elements of the crime, one of which is an intent to kill. And Jane's claim of mistake is nothing more than a denial of that intent.

With respect to the third criticism of the traditional approach, which criticism is directed at the maxim that ignorance of the law is no excuse, consider the case of Regina v. Smith (David), 2 Q.B. 354 (1974). The defendant there was charged with intentionally destroying the property of another. He had torn off some paneling that he himself had earlier affixed to the walls of his apartment. When he tore it off the walls, he believed he had a legal right to do so because he had installed it. He was unaware that the law of property makes items affixed by a tenant to a landlord's property the legal property of the landlord rather than the tenant. Assuming, however, that the criminal statute required knowledge that the property the defendant was destroying belonged to another, the defendant's ignorance of the law would render him not guilty of the crime, even under the traditional approach, and the court so held.

Moreover, the traditional approach to ignorance of law is tempered in the United States by various constitutional doctrines prohibiting ex post facto punishment and punishment under vague laws. And in one instance, Lambert v. California, 355 U.S. 225 (1957), involving a law requiring a specific action that almost no one would anticipate, the Supreme Court held punishment of one who was unaware of the requirement to be unconstitutional.