In the United States’ criminal justice system, we follow a general rule that people are only punished for past wrongful acts that were intentionally performed. The distinction between the actus reus and mens rea goes to the heart of two of those requirements: “wrongful acts” and “intentionally performed.”
The actus reus (literally Latin for ‘guilty act’) is the behavior that the criminal law intends to punish. It can be either an act of commission (such as hitting or stealing) or it might be an act of omission (such as failing to file a tax return or neglecting a child.) In either case, we punish people only when they either do something that is prohibited, or fail to do something that is required of them.
The mens rea (literally Latin for ‘guilty mind’) is the element of a crime that only allows punishment for actions that we voluntarily take. This is NOT, however, the same thing as intending a particular outcome. For example, we generally do not punish people if they do something that they could not control. Imagine that a doctor taps your knee and your lower leg reacts by reflexively kicking forward. Even if kicking another person were a “guilty act,” you would not possess that required “guilty mind” of voluntarily moving your foot until it hit the doctor. Your action was not within your control. This same logic would apply if you were forced to do something completely outside your control. Imagine a town where it is illegal for a person to be on the streets after midnight. Imagine that a group of people break into your house, carry you to the street, and place you in front of a police officer. You did not voluntarily do anything, and you therefore lacked the required mens rea.
This is different from the principle known as criminal intent. That concept refers to how much you intended for a particular result to happen. For example, imagine that the law makes it a crime to intentionally take the life of another person. If you pull the trigger of a gun believing that it is empty, but inadvertently kill another person, you probably did not have the intent to kill them, and likely are not guilty of the crime as defined. But you certainly did have the required mens rea: the action that you performed (pulling the trigger) was voluntary and knowing. You meant to perform the act of pulling the trigger, even if the consequences of taking another person’s life were beyond your intention.