An accused person can escape criminal liability for an offence that they could otherwise be convicted for by sufficiently proving the existence of one or more established defences.
Defences are often categorized as follows:
- Justification: a justifiable act is one that makes out the offence but is justified by the external circumstances in which the act was committed. (e.g. self-defence)
- Excuse: a criminal act is excused where the accused would have committed an undesirable criminal offence, the punishment for the offence would be morally inappropriate due to the extenuating circumstances. (eg. offences due to mental infirmities, duress). Defence such as necessity and duress are excuses since they are founded in their moral involuntariness.
- Affirmative Defence: a defence that, if proven, will eliminate liability despite all the elements of the offence having been made out
- Negating Defence: a defence that negates one more more of the essential elements of the prosecution's case.
By s. 19, the ignorance of the law is no defence. However, there are limited exception where the underlying belief is due to offcially induced error or mistake of fact.
An affirmative defence is generally considered a type of justification defence.
Whether the judge correctly put a defence to a jury is a question of law and reviewable on a standard of correctness.
Under s. 794, the burden is on the defendant to prove an exception or excuse:
No need to negative exception, etc.
794 (1) No exception, exemption, proviso, excuse or qualification prescribed by law is required to be set out or negatived, as the case may be, in an information.
(2) The burden of proving that an exception, exemption, proviso, excuse or qualification prescribed by law operates in favour of the defendant is on the defendant, and the prosecutor is not required, except by way of rebuttal, to prove that the exception, exemption, proviso, excuse or qualification does not operate in favour of the defendant, whether or not it is set out in the information.
R.S., c. C-34, s. 730.
Excuses and justifications "do not negative the mens rea rather they operate by justifying or excusing what would otherwise be criminal conduct".
- R v Hibbert, 1995 CanLII 110 (SCC),  2 SCR 973 (SCC) para 47
- R v Ryan, 2013 SCC 3 (CanLII), per LeBel and Cromwell JJ, at para 23
- Ryan, ibid. at para 23 citing R v Ruzic, 2001 SCC 24 (CanLII)
- Ryan, ibid. at para 23
e.g. Refusal (Offence)
R v Greenshields, 2014 ONCJ 35 (CanLII) at para 16
R v Taraschuk, 1975 CanLII 37 (SCC),  1 SCR 385
see also Intention
Raising a Defence
Types of Defences
- Reasonable Excuse
- Self-Defence and Defence of Another
- Defence of Property
- Acting in Authority
- Officially Induced Error
- Corrective Force
- Accident and Mistake
- De Minimus Non Curat Lex
- Due Diligence
- Alternative Suspect Defence
- Innocent Possession
- Mistaken Belief of Age