When Should a Mistake of Fact Excuse?

This symposium contribution advances two claims. First, a mistake (or ignorance) of fact should matter in the criminal law only insofar as it causes an actor to be ignorant of the law. Mistakes of fact should have exculpatory force if and only if they cause ignorance of the law, which ignorance ought to be an excuse to criminal liability. Second, an actor who should otherwise be excused because he is ignorant of the law, which ignorance is in turn the result of some mistake of fact, should not lose that excuse just because the underlying mistake of fact can in a plausible and legitimate sense be characterized as unreasonable