There can be no doubt that, whatever other function food may or may not have, it replaces broken-down tissue. The tissue-wastes of the preceding day are replenished by the food eaten; so the body remains about the same in weight, no matter how much exercise be taken, or how much tissue is broken down. These tissues are very complex in their nature, and a variety of food is consequently necessary to restore the tissues destroyed—food containing a number of elements (the counterpart of the elements destroyed), being necessary to offset the waste. Proteid, fats, carbohydrates, and various salts are, therefore, necessary in the food; and no food that does not contain these constituents, in larger or smaller quantities, can be used by the body, or can be classed as a true “food.” Other things being equal, therefore, it may be said that _a food is nutritious and capable of sustaining life in proportion to its complexity—the best food being one that most nearly supplies the wastes of the tissues_. If an article of diet contains only one of the essential elements necessary for supporting life, the body, if fed upon it, will waste away and die—no matter _how much_ of that food be eaten. In certain experiments conducted upon dogs, it was found that, when they were fed upon fat—_e.g._ they became round, plump, _embonpoint_, and yet died of inanition! The same would be true of any other single article of diet. If an animal were fed upon it, he would surely die, sooner or later. Proteids are supposed to supply most of the muscle-forming elements, and a part of the energy expended by the body; fats and carbohydrates are supposed to be of use chiefly in supplying heat and energy to the system. The mineral salts that are contained in the foods do not fulfil any definite function, so far as is known; but they are very essential, nevertheless. If a diet lacks these salts entirely, the body wastes away and dies of “saline starvation.” It will thus be apparent that foods very rapidly and very forcibly affect the state of the health, and even the life of the individual. Food, it must be remembered, _makes blood_; and the blood is absolutely dependent upon the food supply for its character and composition. If the food be poisonous in character, the blood soon becomes tainted, and the mind, no less than the body, shows the effects of this poisoning process. On any theory we may hold of the nature of mind, and its connection with the body, it is certainly _dependent_ upon the body for its manifestation, in this life; and is coloured and influenced by the state of the body, and by the condition of the blood. This I have shown more fully in another place. We shall also see the effects of diet upon the mind, more clearly, as we proceed in the present volume.