⇒ It is in fact technically incorrect to see mistake as a special category of defence.

⇒ Rather a mistake can be a particular way of denying the defendant has mens rea or asserting that the defendant has a defence, such as self-defence. Consider these four examples:

1. The mistake negates mens rea → Anna was out shooting rabbits and saw something in the bushes she thought was a rabbit and shot it. It was actually Tom who she shot, and he died. Anna would have a defence of no mens rea to a charge of murder. Her mistake meant that she did not have an intention to kill or cause grievous bodily harm to a person.
2. The mistake does not negagte mens rea → Eva is a terrorist. She shoots Damien dead. Eva believes Damien is a government minister, but is mistaken. Here Eva is guilty of murder, but her mistake did not negate her mens rea (she did intend to kill a person).
3. The mistaken belief forms the basis of a defence → Liz mistakenly believes Henry is about to attack her and so she stabs him dead. Liz will be able to rely on the defence of self-defence.
4. A mistake of law provides no defence (ignorantia juris neminem excusat) → Grace buys cannabis. She misunderstands an inaccurate news report and believes it is no longer an offence to possess cannabis. She is charged with possession of cannabis. It is no defence for her to claim that she thought what she was doing was legal.