The law insists on maintaining mental disorder as a predicate for a wide array of legal provisions, in both the criminal justice system and the civil law. Among adults, only a person with a "mental disease or defect" can escape conviction for an intentional, unjustified crime on grounds of cognitive or volitional impairment.' Only people with "mental illness" or "mental disorder" may be subjected to indeterminate preventive commitment based on dangerousness. Under the laws of many states, only people with a mental disorder are prevented from making decisions about treatment, criminal charges, wills, contracts, and a host of other important aspects of life. What is it about "mental illness" that merits such special legal treatment? Why are mentally ill people singled out by the law in so many different contexts? These questions can be answered only by first figuring out what the law is trying to accomplish in those areas in which it makes mental illness relevant. In undertaking this effort, this article focuses on those settings in which mental illness is a predicate for either avoiding or imposing a deprivation of liberty. It is in these situations, where the stakes are the highest, that precisely determining what the law means and should mean when it uses the term "mental illness" and like terminology is most important. The article starts, in Part I, with a description of how the behavioral sciences have defined mental disorder. The definitions are varied and expansive, leaving the law plenty of working room. Part 11 then looks at how the law treats the concept of mental illness. In general, the law's definitions of this phenomenon have been equally vague, and they are often nonresponsive to its own normative objectives...There are two overarching theses to this article. The first is that the choice as to how mental illness should be defined for legal purposes should be based on pragmatic as well as normative concerns since normative analysis leaves so many questions unanswered. A related thesis is that mental disorder is such a vacuous phrase that the law should consider dispensing with it as an independent criterion for intervention and instead simply identify as precisely as possible the types of mental dysfunction it wants to treat specially.