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Mens rea, or criminal intent, is the essential mental element considered in court proceedings to determine whether criminal guilt is present, while actus reus functions as the essential physical element. In all conventional criminal trials in the United States, these two elements, Latin terms for "culpable mind" and "culpable action" respectively, are required to establish the guilt of a defendant.
While the concept of criminal intent is used the world around, the term mens rea is specific to Anglo-American criminal law. The mens rea concept originated in England in the early 17th century, and is based on the idea that it is morally wrong to punish a person for harm done to society innocently and unwittingly. In order to be guilty, the criminal must have committed his act in a culpable mental state. So while criminal action (actus reus) is an essential element in determining whether a crime actually occurred, a person may be judged not guilty of if mens rea is absent. (An exception to this rule is found in cases of liability without fault.)
Mens rea is applied to specific cases by degrees of magnitude, which are determined by considering a mixture of desire and aforethought. Desire is here not intended in the sense of emotional passion, but rather in the sense of decided resolve. For example, you may not passionately desire to evade your taxes, but you may decide to do it anyway, under some circumstances. The level of desire is weighed against the degree of planning, or aforethought, involved in the criminal act to determine the magnitude of criminal guilt and, by extension, the appropriate severity of punishment.
Mens rea has roughly four different degrees of magnitude, which are agreed on throughout most of the states in the United States. These states of mind are, in descending order of magnitude, "intention" or "purpose," "knowledge," "recklessness" (often called "willful blindness" in the United States), and "gross (or criminal) negligence." The same offense (e.g., killing a man) can incur a wide range of punishment, depending entirely on what level of mens rea is found present in the criminal's mind at the time the crime was committed. The difference between criminal negligence and deliberate intention in this case could be the difference between a few years in prison and capital punishment.