Mistake of Fact

Mistake of Fact

Based on the principle of criminal intent, if the defendant honestly but mistakenly believed certain facts, and that mistaken belief meant that he did not have "an intention to kill or an intention to cause really serious bodily harm", then the prosecution cannot prove the subjective intent required for murder, and the charge of murder may not be made out. 

For example, A, during a stage rehearsal, uses a prop gun that he honestly believes is loaded only with blanks and performs a demonstrative shooting action towards B. In fact, the gun had been loaded with live rounds, and B is shot and dies as a result. Because A honestly believed there were no live rounds in the gun, he could not possibly have had the subjective "intention to kill or to cause grievous bodily harm", and therefore lacked the criminal intent required for murder. 

As another example, A, while caring for B, honestly believes that what he is administering is the ordinary medication prescribed by the doctor, but the medication has in fact been switched by another person for a lethal poison, and B dies after taking it. A held an honest mistaken belief as to the nature of the medication and had no intention to kill or to cause serious harm, so the prosecution would find it difficult to prove the criminal intent required for murder. 

It should be noted that the defendant does not need to "prove" his mistake of fact. So long as there is sufficient evidence in the case to raise the issue, the prosecution must still prove, to the standard of beyond reasonable doubt, that the defendant had the criminal intent required for murder.