Moral rightness and the significance of law: Why, how, and when mistake of law matters

The question of whether a mistake of law should negate or mitigate criminal liability is commonly considered to be pertinent to the culpability of the agent, often examined in light of the (epistemic) reasonableness of the mistake. I argue that this view disregards an important aspect of this question; namely, whether a mistake of law affects the rightness of the action, particularly in light of the moral significance of the mistake. I argue that several plausible premises regarding moral rightness under uncertainty, the nature of law, and the moral significance of law entail a positive answer to this question. Specifically, I consider this argument: (1) one (subjective) sense of moral rightness depends on the (epistemically justified) belief of the agent concerning a non-moral fact that is morally significant; (2) a law is (partly) a non-moral fact; (3) a legal fact might be morally significant; (4) therefore, an action that is compatible with an applicable moral standard, in light of the mistaken (justified) belief of the agent concerning a morally significant law, is (subjectively) right or less wrongful; (5) the (subjective) moral rightness of an action counts against criminal liability for this action; (6) therefore, an action that is compatible with the applicable moral standard, in light of the mistaken (epistemically justified) belief of the agent, counts against criminal liability for the action if the law is morally significant.