"Again, two persons may be exposed to the same atmospheric conditions, or be brought within a similar miasmatic influence, resulting in the one case in fever; in the other, in nothing. The miasm or poison was doubtless present in both systems alike, and, perhaps, in similar quantity, and yet in one case with no appreciable effect, and in the other, creating an illness which may continue for weeks or months. It is evident that there must have been, in the last case, a condition of the system which rendered the action of the miasm possible, and without which, it would have been entirely, or nearly, inert. In fact, strictly speaking, this cause, or antecedent condition, is rather a part of the disease itself, and cannot, in fairness, be separated from it. For, if disease be abnormal action, either in the structure or function of an organ, whether sufficient to be recognized or not, subjectively or objectively, then this weakness, inherited or acquired, becomes the chief factor, and those changes, which are sufficiently gross to be observed by our senses, and which we are accustomed to term disease, are only the remaining factors of it."[3] I apply the same course of reasoning in relation to _that necessarily antecedent condition of the brain_, which renders it especially liable to become diseased through the operation of those ordinary exciting causes, which, to a greater or less extent, surround the lives of all persons, and which we term _the Insane Diathesis_. It becomes necessary, therefore, to study this peculiar condition of the brain, first, as to its _nature_, and, second, as to some of those _causes_ which tend especially to create it. CHAPTER IV. THE INSANE DIATHESIS. The ideal human system would be one of perfection, that is, it would be one so constituted as to discharge all its functions perfectly. Yet, its capacities would be limited as they now are, though not to the same extent. Digestion of such articles of food as the system requires would be perfect, though this might not be true as to many other articles, which are appropriate as food for other animals. Sight and hearing would be perfect, but only within certain ranges and distances; memory would be perfect in reference to every thing comprehended and understood. The limitations would be dependent on the inherent nature of the organism, in its relation to the external world. What is stated above as true, in relation to certain parts of the system, would be equally true of the functions of all parts of the human system, both physical and psychical.