269. Every one is guilty of an indictable offence and liable to two years' imprisonment who...

(b) assaults any public or peace officer engaged in the execution of his duty, or any person acting in aid of such officer.

The problem: There is no mens rea for the circumstances component of the actus reus.

So: Must one have known that the person one assaulted was a police officer? Or is the requirement something less? Or is this a strict or absolute liability offence?

The solution: The Court "reads in" the requirement of knowledge.

1. Other cases (stare decisis). McLeod is like Sherras v. De Ruzen, and not like R. v. Prince.

2. Common law principles.

(a) Actus non facit reum nisi mens sit rea.
(b) Everyone is entitled to a fair trial, and so to the right to raise any reasonable defence.

If a statute doesn't contain a mens rea requirement, courts cannot read one into the material elements. But courts may permit a defence of the absence of mens rea, to be proven by the defendant on the balance of probabilities.

(See Q1 and A1 in Questions and Answers for a clarification of this point.)

This has changed: now the Crown must prove all the elements, even those read in by the Courts.