Actus reus non facit reum nisi mens sit rea: (Latin) an act does not make a defendant guilty without a guilty mind.
Latin for Lawyers (London: Sweet & Maxwell, 1960) has it:
"The act itself does not constitute guilt unlerss done with a guilty intent."
In R v Tolson (23 QBD): "Ordinarily speaking, a crime is not committed if the mind of the person doing the act in question be innocent".
And in Fowler v. Padget (1798) 101 ER 1103:
"It is a principle of natural justice, and of our law, that actus non facit reum nisi mens sit rea. The intent and the Act must both concur to constitute the crime."
This Latin phrase is often given as a pinnacle of the common law criminal justice system, and usually in the context of mens rea rather than actus reus.
In his History of the Criminal Law (1883), James Stephen wrote:
"The maxim is sometimes said to be a fundamental principle of the whole criminal law, but I think that, like many other Latin sentences supposed to form part of the Roman law, the maxim not only looks more instructive than it really is, but suggests fallacies which it does not precisely state. It is frequently, though ignorantly, supposed to mean that there cannot be such a thing as legal guilt where there is no moral guilt, which is obviously untrue, as there is always a possibility of a conflict between law and morals."
And as is quite rightly pointed out in a 1930 article in 52 CCC 202, entitled What Is Crime?:
"Ordinarily ... mens rea is an essential ingredient of a crime. But when the legislature expressly declares an act to be criminal, the question of intention or malice need not be considered except as affecting the quantum of punishment. A statute may be so framed as to relate to such a subject-matter and make an act criminal whether there has been any intention to break the law or otherwise to do wrong or not.
"The legislature has power to make the bare doing of a particular act a crime, no matter how innocent from a mental point of view the doer of it may be; in such a case the doer must be held to be a criminal."