This reason is worthy of the doctrine it is used to uphold; as if a plea of ignorance, any more than any other plea, must necessarily be believed simply because it is urged; and as if it were not a common and every-day practice of courts and juries, in both civil and criminal cases, to determine the mental capacity of individuals; as, for example, to determine whether they are of sufficient mental capacity to make reasonable contracts; whether they are lunatic; whether they are _compotes mentis_, "of sound mind and memory," &c. &c. And there is obviously no more difficulty in a jury's determining whether an accused person knew the law in a criminal case, than there is in determining any of these other questions that are continually determined in regard to a man's mental capacity. For the question to be settled by the jury is not whether the accused person knew the particular _penalty_ attached to his act, (for at common law no one knew what penalty a _jury_ would attach to an offence,) but whether he knew that his act was _intrinsically criminal_. If it were _intrinsically criminal_, it was criminal at common law. If it was not intrinsically criminal, it was not criminal at common law. (At least, such was the general principle of the common law. There may have been exceptions in practice, owing to the fact that the opinions of men, as to what was intrinsically criminal, may not have been in all cases correct.) A jury, then, in judging whether an accused person knew his act to be illegal, were bound first to use their own judgments, as to whether the act were _intrinsically_ criminal. If their own judgments told them the act was _intrinsically_ and _clearly_ criminal, they would naturally and reasonably infer that the accused also understood that it was intrinsically criminal, (and consequently illegal,) unless it should appear that he was either below themselves in the scale of intellect, or had had less opportunities of knowing what acts were criminal. In short, they would judge, from any and every means they might have of judging; and if they had any reasonable doubt that he knew his act to be criminal in itself, they would be bound to acquit him. The second reason that has been offered for the doctrine that ignorance of the law excuses no one, is this: "Ignorance of the municipal law of the kingdom, or of the penalty thereby inflicted on offenders, doth not excuse any that is of the age of discretion and compos mentis, from the penalty of the breach of it; because every person, of the age of discretion and compos mentis, _is bound to know the law_, and presumed to do so. _Ignorantia eorum, quæ quis scire tenetur non excusat_." (Ignorance of those things which every one is bound to know, does not excuse.)--_1 Hale's Pleas of the Crown_, 42. _Doctor and Student, Dialog. 2_, ch. 46. _Law Magazine_, (_London_,) vol. 27, p. 97.