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This is a small sample of the definition for Mens Rea in Dean's Law Dictionary.

Latin. Guilty mind. A defendant's state of mind at the time he commits the crime. Mens rea is generally divided into four distinct elements of intent that relate to the proscribed actus reus: 1) This intent. 2) Less than this intent. 3) More than this intent. 4) A combination of intents 1,2, or 3.

It is one of the most fundamental postulates of our criminal justice system that conviction can result only from a violation of clearly defined standards of conduct. Our law does not punish bad purpose standing alone, however; instead we require that mens rea accompany the actus reus specifically proscribed by statute.' As the Supreme Court has recognized, William Shakespeare's lines here illustrate sound legal doctrine. His acts did not o'ertake his bad intent; And must be buried but as an intent That perish'd by the way: thoughts are no subjects, Intents but merely thoughts. United States v. Apfelbaum, 445 U.S. 115, 131 n.13, 63 L. Ed. 2d 250, 100 S. Ct. 948 (1980) (quoting William Shakespeare's Measure for Measure, Act V, Scene 1; G. Williams, Criminal Law, The General Part 1 (2d ed. 1961)).

The contention that an injury can amount to a crime only when inflicted by intention is no provincial or transient notion. It is as universal and persistent in mature systems of law as belief in freedom of the human will and a consequent ability and duty of the normal individual to choose between good and evil. For a brief history and philosophy of this concept in Biblical, Greek, Roman, Continental and Anglo-American law, see Radin, Intent, Criminal, 8 Encyc. Soc. Sci. 126. For more extensive treatment of the development in English Law, see 2 Pollock and Maitland, History of English Law, 448-511. 'Historically, our substantive criminal law is based upon a theory of punishing the vicious will. It postulates a free agent confronted with a choice between doing right and doing wrong and choosing freely to do wrong.' Pound, Introduction to Sayre, Cases on Criminal Law (1927).

A relation between some mental element and punishment for a harmful act is almost as instinctive as the child's familiar exculpatory 'But I didn't mean to,' and has afforded the rational basis for a tardy and unfinished substitution of deterrence and reformation in place of retaliation and vengeance as the motivation for public prosecution. In Williams v. New York, 337 U.S. 241, 248, the Court observed that 'Retribution is no longer the dominant objective of the criminal law. Reformation and rehabilitation of offenders have become important goals of criminal jurisprudence.' We also there referred to '. . . a prevalent modern philosophy of penology that the punishment should fit the offender and not merely the crime.' Id., at 247. Such ends would seem illusory if there were no mental element in crime. Unqualified acceptance of this doctrine by English common law in the Eighteenth Century was indicated by Blackstone's sweeping statement that to constitute any crime there must first be a 'vicious will.' 4 Bl. Comm. 21. .....