<P> Mens rea varies depending on the offense . For murder, the mental element requires the defendant acted with "malice aforethought". Others may require proof the act was committed with such mental elements such as "knowingly" or "willfulness" or "recklessness". Arson requires an intent to commit a forbidden act, while others such as murder require an intent to produce a forbidden result . Motive, the reason the act was committed, is not the same as mens rea and the law is not concerned with motive . </P> <P> Although most legal systems recognize the importance of the guilty mind, or mens rea, exactly what is meant by this concept varies . The American Law Institute's Model Penal Code has reduced the mental states to four . In general, guilt can be attributed to an individual who acts "purposely," "knowingly," "recklessly," or "negligently ." Together or in combination, these four attributes seem basically effective in dealing with most of the common mens rea issues . </P> <P> All crimes require actus reus . That is, a criminal act or an unlawful omission of an act, must have occurred . A person cannot be punished for thinking criminal thoughts . This element is based on the problem of standards of proof . How can another person's thoughts be determined and how can criminal thoughts be differentiated from idle thoughts? Further, the law's purview is not to punish criminal ideas but to punish those who act upon those ideas voluntarily . </P> <P> Unlike thoughts, words can be considered acts in criminal law . For example, threats, perjury, conspiracy, and solicitation are offenses in which words can constitute the element of actus reus . </P>

Actus reus means the guilty act and is one element the prosecution must prove in a criminal case