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.

From Supreme Court judgements:

++ Penalty, a civil obligation and a coercive remedy for loss of revenue - In contradistinction, criminal sentence not imposable unless element of mens rea established

++ If statute forbids an act, doing it itself supplies mens rea and it is for the suspect to bring himself within statutory defences, if any

++ A person who knowingly purchases smuggled goods from an importer cannot have an intention to evade a prohibition against import, for the prohibited goods have already been imported.

++ Mens Rea is an essential ingredient of a criminal offence.