Mens rea is Latin for "guilty mind". In criminal law, it is viewed as 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 "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). As a general rule, criminal liability does not attach to a person who acted with the absence of mental fault. The exception is strict liability crimes.

In civil law, it is usually not necessary to prove a subjective mental element to establish liability for breach of contract or tort, for example. However, if a tort is intentionally committed or a contract is intentionally breached, such intent may increase the scope of liability as well as the measure of damages payable to the plaintiff.

In Roman mythology, Mens, also known as Bona Mens or Mens Bona (Latin for "Good Mind"), was the personification of thought, consciousness and the mind, and also of "right-thinking". Her festival was celebrated on June 8. A temple on the Capitoline Hill in Rome was vowed to Mens in 217 BC on advice from the Sibylline Books, and was dedicated in 215 BC.

The Latin word mens expresses the idea of "mind" and is the origin of English words like mental and dementia. The gifted-only organization Mensa International was originally to be named mens in the sense of "mind", but took instead the name Mensa (Latin: "table") to avoid ambiguity with "men's" in English and "mens" in other languages.

REA or Rea may refer to: