7.4 The Structure of a Crime
7.4.1 The Actus Reus and Mens Rea Dichotomy
Although penal laws differ greatly from country to country, it is nevertheless
possible to discover on a doctrinal level some striking similarities among different
criminal justice systems. It seems, for instance, to be a general principle of law that
the attribution of liability generally requires an analysis of two aspects. Each crime
can be split intoactus reus, the objective element of a crime, andmens rea, the
mental or subjective element of the crime. The offense of murder is, for instance,
often defined as the intentional killing of another human being. In this case, the
actus reusconsists of killing another person, while themens reaelement of this
offense requires that the perpetrator did so intentionally. In our opening case, it
seems clear that theactus reusrequirement of murder has been fulfilled as Mrs.
M. has been killed by the poisoned cake. But how about themens rearequirement?
Did the defendant really intend to kill Mrs. M.? This seems more difficult to
establish, but as we will discover later on, it all depends on how intention is defined
in a legal system (see Sect.7.6).
In any case, in order to be liable for murder, a perpetrator needs to fulfill both the
mens reaand theactus reusrequirements. This legal demand often finds expression
in the famous Latin phraseactus non facit reum nisi mens sit rea, which can loosely
be translated as “an act does not make a man guilty unless his mind is (also) guilty.”
This important dichotomy in criminal law arguably stems from the distinction
between the objective or tangible side of a person’s conduct, which is susceptible to
objective assessment, and the intangible, subjective side of a person’s conduct,
i.e. his state of mind, which is not. Furthermore, as modern criminal law has its
roots in the tradition of the Enlightenment, the very effort to distinguish between
objective and subjective elements of criminality rests on the old Cartesian concep-
tion that human beings consist of two separate elements, i.e. mind and body.
In order to be held criminally liable, the elements generally need to be present
simultaneously, as a person cannot be held liable in a liberal society for a conduct
that he did not intend and at most contemplated (thoughts are free). This basic
distinction betweenactus reusandmens reais however not a hard and fast one, and
it should be kept in mind that the two notions are best viewed as conceptual tools
under the umbrella of which a multitude of different doctrines are pigeonholed.
Under the heading ofmens rea, for instance, the different gradations of intention,
recklessness, and negligence are frequently discussed (see Sect.7.6). Theactus reus
element, on the other hand, is generally considered to include the doctrine of
conduct, including omissions (see Sect.7.5), as well as the doctrine of causation.
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