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In criminal law, mens rea -- the Latin term for "guilty mind" -- is usually one of the necessary elements of a crime. The standard common law test of criminal liability is usually expressed in the Latin phrase, actus non facit reum nisi mens sit rea, which means that "the act does not make a person guilty unless the mind is also guilty". Thus, in jurisdictions with due process, there must be an actus reus accompanied by some level of mens rea to constitute the crime with which the defendant is charged (see the technical requirement of concurrence). The exception is strict liability crimes (in the civil law, it is not usually necessary to prove a subjective mental element to establish liability, say for breach of contract or a tort, although if intentionally committed, this may increase the measure of damages payable to compensate the plaintiff).
Quite simply, therefore, mens rea refers to the mental element of the offence that accompanies the actus reus. In some jurisdictions, the terms mens rea and actus reus have been superseded by alternative terminology. In Australia, for example, the elements of all Federal offences are now designated as "Fault Elements" (mens rea) and "Physical Elements" (actus reus). This terminology was adopted in order to replace the obscurity of the Latin terms with simple and accurate phrasing.
There are four general classes of mens rea (the words used may vary from one state to another and from one definition to another) but the substance is: