Beaver v The Queen [1957] is a leading decision of the Supreme Court of Canada on the mens rea requirement in criminal law to prove "possession".

In defence, Louis claimed that he thought the package was milk sugar and that they were only trying to defraud the RCMP officer.

The issue before the Supreme Court was whether a conviction based on possession requires knowledge of the nature of the object.

Cartwright J., writing for the majority stated that it is a fundamental principle of criminal law that the Mens rea of an element of an offence must be proven to secure a conviction.

It has been established that provisions in the Drug Act are criminal law, and that any offence that allows a punishment of prison requires proof of mens rea.