Actus reus is the act—or the omission or failure to act when there is a duty to act—that comprises the physical elements of the commission of a crime. The actus reus generally must be voluntary bodily movements that constitute a crime.
In contrast, mens rea (meaning “guilty mind”) is the criminal intent element of a crime. It is the intent to commit a crime, or the knowledge that the action, or failure to act when there is a duty to act, will result in the commission of a crime.

In Massachusetts, as in other jurisdictions, the concept of actus reus requires that a defendant has engaged in a voluntary act, or an omission when there is a legal duty to act, that constitutes a breach of the law. This means that the physical conduct of the defendant must be a conscious and voluntary action that leads to the commission of a crime. For instance, if someone is charged with theft, the actus reus would be the taking of someone else's property. On the other hand, mens rea refers to the mental state of the defendant at the time of the crime. Massachusetts law recognizes various levels of mens rea, including intent, knowledge, recklessness, and negligence, depending on the crime. The prosecution typically must prove that the defendant had the requisite mens rea to commit the crime in question. For example, in a case of intentional murder, the prosecution must show that the defendant had the intent to kill or cause grievous bodily harm. Both actus reus and mens rea are critical components in establishing criminal liability under Massachusetts law.