This book argues that ignorance of law should usually be a complete excuse from criminal liability. It defends this conclusion by invoking two presumptions: first, the content of criminal law should conform to morality; second, mistakes of fact and mistakes of law should be treated symmetrically. The author grounds his position in an underlying theory of moral and criminal responsibility according to which blameworthiness consists in a defective response to the moral reasons one has. Since persons cannot be faulted for failing to respond to reasons for criminal liability they do not believe...