The presumption of sanity is a concept in criminal law used to allocate burdens relating to the insanity defense. In Clark v. Arizona, the U.S. Supreme Court relied on the presumption to affirm the exclusion of evidence introduced to negate mens rea. This Comment discusses the problems with using the presumption to exclude evidence from mens rea determinations and examines other ways to regulate mental disease evidence, which require modifying Justice Souter’s categorization of such evidence in Clark. This Comment argues that mental disease evidence should be admissible if it shows a potential mistake of fact that bears directly on statutory intent. This form of defense—showing failure of proof of a statutory element—is conceptually distinct from an affirmative defense of legal insanity. The latter deals with whether a defendant is legally responsible for a crime, rather than whether the defendant committed the offense charged. The presumption of sanity relates solely to the insanity defense; it has no bearing on statutory intent and should not be used to exclude evidence from such a determination.