Actus Reus

There are two contrasts with other elements of criminal liability that help to clarify the nature of actus reus. The first is the contrast with mens rea. Mens rea literally translated from the Latin means guilty mind. The technical legal use of the phrase denotes that prerequisite of criminal liability having to do with the state of mind of the accused when he committed the actus reus of some offense. Thus, one of the mens reas sufficient for murder is general intent: such requirement is often stated as a prohibition on "intentionally killing another human being." The word "intentionally" tells us what kind of mental state an accused must have to be guilty of this kind of murder (either an intent or a belief, as it turns out). The phrase "killing another human being" tells us two things: first, what must be done by way of action to be guilty of murder; and second, what object an accused's intention or belief must take in order to be guilty of murder (Moore, 1993). The first is the actus reus requirement, whereas the second is part of the mens rea requirement. The accused must both actually kill someone, and intend (or believe) that he is killing someone, in order to be guilty of this kind of murder.

The relationship between actus reus and mens rea is not always this close in all offenses. In what are often called specific intent offenses, for example, the object of the prohibited mens rea will not coincide with the act prohibited by law. Thus, the actus reus of common law burglary is the breaking and entering of the dwelling house of another at night, whereas the mens rea includes the requirement that the accused do such breaking and entering with the intent to commit a felony once inside. The commission of such a further felony is no part of the actus reus of burglary, but the intent to commit such a further felony is part of the mens rea of burglary.

In its actus reus/mens rea distinction the criminal law has mirrored a deep divide in morality. This is the divide between wrongdoing and culpability. Although it is disputed, morality is most often thought to contain certain prohibitions and requirements, such as "Do not kill" and "Help others in distress." Morality generally permits us either to do or to refrain from doing most acts, but morality forbids certain actions and requires others. To do an act morality forbids, or to refrain from doing an act morality requires, is to breach one's moral obligations. This is moral wrongdoing.

Morality likewise concerns itself with the culpability with which a wrongful act is done. Overall moral blameworthiness includes culpability as well as wrongdoing. One is free from moral blame for causing a harm to another if one neither intended to cause such a harm, believed one's act could result in such a harm, or unreasonably risked such a harm coming about because of one's actions.

The legal distinction between actus reus and mens rea is best seen as a reflection of this underlying moral distinction. The parallel is one of form, with criminal law and morality dividing criminal liability and moral responsibility (respectively) into these two elements. The difference, of course, lies in the content of legal versus moral norms; in many legal systems much that morality prohibits or requires the law does not, and vice versa.