Mens rea literally, guilty mind, in law Latin. It is one of the two main prerequisites (along with actus reus) for prima facie liability to criminal punishment in Anglo-American legal systems. To be punishable in such systems, one must not only have performed a legally prohibited action, such as killing another human being; one must have done so with a culpable state of mind, or mens rea. Such culpable mental states are of three kinds: they are either motivational states of purpose, cognitive states of belief, or the non-mental state of negligence.
To illustrate each of these with respect to the act of killing: a killer may kill either having another’s death as ultimate purpose, or as mediate purpose on the way to achieving some further, ultimate end. Alternatively, the killer may act believing to a practical certainty that his act will result in another’s death, even though such death is an unwanted side effect, or he may believe that there is a substantial and unjustified risk that his act will cause another’s death. The actor may also be only negligent, which is to take an unreasonable risk of another’s death even if the actor is not aware either of such risk or of the lack of justification for taking it. Mens rea usually does not have to do with any awareness by the actor that the act done is either morally wrong or legally prohibited. Neither does mens rea have to do with any emotional state of guilt or remorse, either while one is acting or afterward. Sometimes in its older usages the term is taken to include the absence of excuses as well as the mental states necessary for prima facie liability; in such a usage, the requirement is helpfully labeled ‘general mens rea,’ and the requirement above discussed is labeled ‘special mens rea.’ See also DIMINISHED CAPACITY, ETHICS, INTENTIO. M.S.M.