The line separating mental health from mental illness is not always clear and distinct. Underlying the debate about what should be considered a mental illness is the question of whether mental illnesses are in fact medical diseases.

Several studies appear to support the argument that mental illnesses are medical diseases. For example, one study found definite differences between the brains of the mentally ill and the mentally healthy. According to psychiatrist Sheldon H. Preskorn, brain-imaging studies of mentally ill patients have demonstrated lesions and other disturbances in the brain structure and/or functions as well as biochemical changes that are not visible in the brains of individuals who do not show symptoms of mental illness. Further proof of the medical basis of mental illness, Preskorn contends, is the fact that treatment is effective for many mental illnesses.

However, critics of this...