The author contends that none of the rationales for not allowing ignorance of the law as an excuse in criminal law cases is persuasive. The paper begins by analyzing the condition under which "reasonable" ignorance of the law ought to be allowed as an excuse. Second, the author indicates in greater detail the sense in which 'justice' requires that we recognize these conditions. Third, the author critically examines the arguments used by legal theorists for disregarding the claims of justice to (...) the individual defendant in cases where reasonable ignorance of law is alleged. Finally, the author wants to show that philosophy matters. A philosophical reading of ignorantia juris non excusat (ignorance of the law does not excuse) can expose conceptual errors that are not seen in legal readings. (shrink)