AbstractSteps in the logical classification of disease. The task of the psychiatrist contrasted with the alienist's. Insanity not an entity; neither are all mental diseases sui generis. Disease–group diagnosis in psychiatry, analogous to botanical classification of “orders.” Ten disease–groups or “orders” plus an eleventh for the residuum of cases. Osis and–acew; Rosa gallica and Neurosyphilis paretica. The key principle. Special discussion of the eleven groups, syphilitic, feeble–minded, epileptic, alcohol, drug and poison, focal brain (incl. traumatic and arteriosclerotic), somatic, senescent–senile, dementia prsecox, manic–depressive, psychoneurotic, and psychopathic. Conclusions.