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We all make mistakes—even criminals. However, suppose someone who thinks that what he is doing is legal turns out to be mistaken, and the act is a crime. Is he guilty? The common law answered this question as it often does: “It depends.” Consider a factual mistake. As a general rule, if Angelica reasonably thinks the white powder in her vial is salt though it is really cocaine, she is not guilty of transporting cocaine. The law treats legal mistakes, however, strikingly differently. Arthur has been told by a state EPA director that he may, without a permit, dump what he knows to be toluene. The advice turns out to be a misinterpretation of the environmental statutes. Such a mistake would never exculpate. This tension between legal and factual mistakes and the exceptions to these general rules create ulcers in law students—not to mention in clients.[1]

The Model Penal Code takes a somewhat different, more subjective view. Angelica’s factual mistake will exculpate her unless the statute punishes “criminally negligent transportation,” and then only if her mistake was a “gross deviation” from the RPP’s standard of care. Arthur’s legal mistake, if constituting a reasonable reliance upon the agency’s advice, would likely be a defense under the Code.


Perhaps no rule of criminal law is better known than the doctrine ignorantia lexis non exusat—“ignorance of the law is no excuse.” Thus, in the example in the Overview, even if Arthur has gone to five lawyers, four priests, three government officials in charge of pollution control, and read the statute books himself, he is still liable if the advice he has received has been erroneous. He will be convicted and punished as though he were just as culpable as Dave, a midnight dumper who dumped toluene in the river, knowing it was illegal and dangerous.

Supporters of the ignorantia rule argue that people should know the law and not act until they do. They argue, further, that anyone could claim reliance on the advice of others, and that this would either be too hard to (dis)prove or generate collusion between defendants and others, who would claim to have given such advice. (As one writer said nearly three centuries ago, “Ignorance of the law excuses no man; not that all men know the law, but because it is an excuse every man will plead, and no man can tell how to confute him.” J. Selden, Table Talk—Law 61 (3d ed. 1716).) A more recent argument sustaining part of the doctrine is that persons who are, or should be, aware that their conduct might be regulated have a “duty to inquire” about the law and are morally blameworthy for failing to ascertain its reach.

Opponents of the doctrine contend that failure to know every statute and administrative regulation, and the interpretation of every statute and administrative regulation, does not reflect moral blameworthiness. (Indeed, if it did, every lawyer, indeed every judge on every court, should beware.) A person who is truly ignorant or mistaken about whether his conduct is unlawful, particularly one who has actively and fairly sought to determine the law, is neither morally culpable (in the “traditional” sense of mens rea) nor purposeful or reckless about breaking the law (in the “statutory” sense of that term).

[1] For a careful and nuanced discussion of the various kinds of mistake, see Kenneth W. Simons, Ignorance and Mistake of Criminal Law, Noncriminal Law and Fact, 9 Ohio St. J. Crim. L. 487 (2012).

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