The terms actus reus and mens rea developed in English Law are derived from the principle stated by Edward Coke, namely, actus non facit reum nisi mens sit rea, which means: "an act does not make a person guilty unless (their) mind is also guilty"; hence, the general test of guilt is one that requires proof of fault, culpability or blameworthiness both in thought and action.
In order for an actus reus to be committed there has to have been an act. Various common law jurisdictions define act differently but generally, an act is a "bodily movement whether voluntary or involuntary." In Robinson v. California, 370 U.S. 660 (1962), the U.S. Supreme Court ruled that a California law making it illegal to be a drug addict was unconstitutional because the mere status of being a drug addict was not an act and thus not criminal. Commentator Dennis Baker asserts: "Although lawyers find the expression actus reus convenient, it is misleading in one respect. It means not just the criminal act but all the external elements of an offence. Ordinarily, there is a criminal act, which is what makes the term actus reus generally acceptable. But there are crimes without an act, and therefore without an actus reus in the obvious meaning of that term. The expression “conduct” is more satisfactory, because wider; it covers not only an act but an omission, and (by a stretch) a bodily position. The conduct must sometimes take place in legally relevant circumstances. The relevant circumstances might include consent in the case of rape. The act of human sexual intercourse becomes a wrongful act if it is committed in circumstances where one party does not consent and/or one or more parties concerned are below the age of consent. Other crimes require the act to produce a legally forbidden consequence. Such crimes are called result crimes. ... All that can truly be said, without exception, is that a crime requires some external state of affairs that can be categorized as criminal. What goes on inside a person's head is never enough in itself to constitute a crime, even though it might be proven by a confession that is fully believed to be genuine."