Many of the modern challenges of mens rea, the mental state element of crime, stem from the purported ambiguities of legislation.1 Sometimes the issue involves uncertainty in interpreting a specified mental state—whether, for instance, “willfully” requires an appreciation that one is violating the law, thereby allowing claims of ignorance through a mistake-of-law defense. At other times, a mens rea requirement may raise questions as to its application within a statute, like whether to distribute an explicit mental state across the elements of a crime. But when these interpretive possibilities prove impossible as a matter of language or reason, or when a provision is entirely bereft of any culpable mental state, the issue takes on a distinctly normative character: whether to infer a mental state requirement based on a free-floating presumption in favor of mens rea. The presumption is born of a belief that mens rea is not just a matter of legislative grace in enacting criminal statutes. At the same time, it is tempered by a profound judicial ambivalence about imposing constitutional constraints on criminal culpability. The mens rea presumption thus exists in the space between statutory law and constitutional law, providing an example of what I will call “mezzanine law.” This essay sketches the concept and its application to some contemporary problems of culpability.