Food replenishes waste caused by the continual wear and tear incidental to daily life: the wear and tear of the muscles in all physical exertion, of the brain in thinking, of the internal organs in the digestion of food, in all the intricate processes of metabolism, in the excretion of waste matter, and the secretion of vital fluids, etc. The ideal diet is one which replenishes waste with the smallest amount of suitable material, so that the system is kept in its normal condition of health at a minimum of expense of energy. The value, therefore, of some general knowledge of the chemical constituents of food is obvious. The diet must be properly balanced, that is, the food eaten must provide the nutrients the body requires, and not contain an excess of one element or a deficiency of another. It is impossible to substitute protein for fat, or _vice versa_, and get the same physiological result, although the human organism is wonderfully tolerant of abuse, and remarkably ingenious in its ability to adapt itself to abnormal conditions. It has been argued that it is essentially necessary for a well-balanced dietary that the variety of food be large, or if the variety is to be for any reason restricted, it must be chosen with great discretion. Dietetic authorities are not agreed as to whether the variety should be large or small, but there is a concensus of opinion that, be it large or small, it should be selected with a view to supplying the proper nutrients in proper proportions. The arguments, so far as the writer understands them, for and against a large variety of foods, are as follows:-- If the variety be large there is a temptation to over-feed. Appetite does not need to be goaded by tasty dishes; it does not need to be goaded at all. We should eat when hungry and until replenished; but to eat when not hungry in order to gratify a merely sensual appetite, to have dishes so spiced and concocted as to stimulate a jaded appetite by novelty of taste, is harmful to an extent but seldom realised. Hence the advisability, at least in the case of persons who have not attained self-mastery over sensual desire, of having little variety, for then, when the system is replenished, over-feeding is less likely to occur. In this connection it should be remembered that in some parts of the world the poor, although possessing great strength and excellent health, live upon, and apparently relish, a dietary limited mostly to black bread and garlics, while among ourselves an ordinary person eats as many as fifty different foods in one day.[3]