Mens rea and actus reus

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The JC’s guide to pithy Latin adages
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Mens rea” is a “guilty mind”, from the axiomatic Latin expression which founds the English criminal law, actus non facit reum nisi mens sit rea:[1] “An act is not guilty unless the mind is guilty”. Its phyiscal counterpart is “actus reus,” the “guilty act”, though of course the pointy of this maxim is that, by itself, the act isn’t guilty. It is only a dirty mind that makes it so.

What counts as a sufficiently guilty mind will depend on the offence, or wrong, in question. For criminal acts, it will generally be intention or recklessness; for strict liability offences and torts, more likely no more than gross negligence or negligence. Only the blamelessly inadvertent get away unscathed, and even they can fall prey to offences of absolute liability.

It is sometimes forgotten by commercial lawyers that the law of contract is very, very different. Contract cares not about your fragile mental state. Your act — performance — is all there is. There is a bit of an essay about this at, of all places, our essay about Section 3(d) Representations under the ISDA Master Agreement.

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  1. You see this often articulated as “actus reus non facit reum nisi mens sit rea,” but with my rudimentary grasp of Latin, that is a reus too far.