A fundamental principle of Criminal Law is that a crime consists of both a mental and a physical element. Mens rea, a person’s awareness of the fact that his or her conduct is criminal, is the mental element, and actus reus, the act itself, is the physical element.
The concept of mens rea developed in England during the latter part of the common-law era (about the year 1600) when judges began to hold that an act alone could not create criminal liability unless it was accompanied by a guilty state of mind. The degree of mens rea required for a particular common-law crime varied. Murder, for example, required a malicious state of mind, whereas Larceny required a felonious state of mind.
Today most crimes, including common-law crimes, are defined by statutes that usually contain a word or phrase indicating the mens rea requirement. A typical statute, for example, may require that a person act knowingly, purposely, or recklessly.