To be guilty of a crime, within most legal systems, there must have been proscribed behavior, known as the actus reus (e.g., taking someone’s property), committed with a prohibited “state of mind,” known as the mens rea (e.g., acting dishonestly). This explanation is sufficient for many purposes, but it can be much more complicated. For example, the criminal act, or behavior, can sometimes be a failure to act (i.e., liability for an omission). And no translation of mens rea is entirely without problems.

Many crimes do not require a mens rea (criminal state of mind). These tend to be less serious crimes. Committing the act, or behavior, is sufficient (e.g., driving at excess speed). These crimes are often known as strict liability offences. But it can ...