“The terms manic-depressive insanity and dementia præcox were used by Kraepelin to designate two disease entities, which he considered were between them responsible for most of the states of mental disorder usually gathered together under the title, the psychoses.” The psychoses are something more than states of mental disorder, and something less than disease entities; they lie between them. Excitement, depression, delirium, and stupor are states of mental disorder which may arise during the course of many diseases, general paralysis, hysteria, epilepsy, the cerebropathies, constitutional and infectious diseases; but acute mania, acute melancholia, anergic stupor, delirious mania, are psychoses. They differ from a state of mental disorder in so far as they are self-sufficient, and are not the expression of an underlying disease; moreover, they run a fairly definite course, ending either in recovery or in dementia. The classifications of the psychoses have been unsatisfactory, and none of them has met with general acceptance. The most satisfactory method is that formulated by Kraepelin: and, in the opinion of Dr. Marshall, he “has done for the psychoses what Erb did for the amyotrophies.” He emphasised the importance of dementia as a termination of the psychoses, and gathered those which ended in dementia into one disease category, dementia præcox, and those which did not so end into another category, manic-depressive insanity. The fact that dementia occurred predicated an organic change in the brain, so that dementia præcox was an organic and manic-depressive a functional disease of the brain. Certain states of mental disorder are common to both conditions: yet there are symptoms which render it possible to distinguish between them. In dementia præcox there is “inco-ordination of the individual psychical processes”: manic-depressive insanity depends on “a change in the mutability of the individual psychical processes.” Normal mentality results from the co-ordinated action of the emotional, intellectual, and volitional processes, and is characterised by a certain congruity of thought and conduct. If there is inco-ordination of these fundamental processes, incongruity of thought and conduct results. The nature of the incongruity depends on the mental process mainly responsible for the incoordination. The symptoms may be for a time emotional, intellectual, or volitional.