In law, knowledge is one of the degrees of mens rea that constitute part of a crime.
[1] Under the principle of ignorantia juris non excusat, ignorance of or mistake about the law is no defense.
[4] For example, in R v. Williams[5] the defendant intervened in what he thought was a mugging but was in fact a citizen's arrest.
It was found that the correct test was whether D "honestly believed" facts which, if true, would establish a defence.
[4] A common example is a person who purchases significantly inexpensive and unprovenanced but desirable items from a stranger.