Abstract

Whether we understand it descriptively or normatively, the slogan that ignorance of the law is no excuse is false. Our legal system sometimes excuses those who are ignorant of the law on those grounds and should. Still, the slogan contains a grain of truth; mistakes of law excuse less readily than mistakes of fact, and ought to. This paper explains the asymmetry by identifying a principle of excuse of the form “If defendant D has a false belief that p and _____, then D is excused”, which has the following feature: it is true frequently when p is a non-legal proposition, but it is false often when p is a proposition about the law. Under this principle of excuse, mistakes excuse by showing the agent to have acceptable commitments for recognizing, weighing, and responding to reasons. Many mistakes of fact show this; they show that the agent’s deliberation led to objectionable action because of faulty inputs and not to fault in the deliberation itself. Mistakes of law, by contrast, frequently indicate that the agent has faulty commitments when it comes to legal reasons; they therefore do not provide excuse under the proposed principle of excuse. It is argued that this explanation of the asymmetry between mistakes of fact and law takes us a great distance towards explaining the relevance of mental state to responsibility, an issue of great importance to moral philosophy.