The most interesting subject in the whole realm of human thought is life. One of the first things that an observing person notices is that the whole world may be divided into two realms: First, the realm of the living; second, the realm of the non-living. One picks up a clam and a pebble on the beach. The clam is living, the pebble is non-living. One glances at the moon shining through the foliage of a tree. The tree is living, the moon is nonliving. One glances at his own body. His body possesses life, but the clothing which covers it does not possess life.

To the child the most interesting thing in the world is his own body, after he becomes conscious of it, and Nature endows every child with the instinct of self-preservation. One of the first things that the infant does is to seek for nourishment. This is purely instinctive and remains instinctive for months; even years later he is hardly conscious of the real significance of his periodical quest for food.

The instinct of self-preservation includes both defense of the body against danger and the quest for nourishment. While these two primary instincts seem to be of equal importance from a philosophical standpoint, the fact remains that we devote vastly more time and energy to the nourishment of our bodies than we do to their defense against danger. Thus, from a practical standpoint, the nourishment or the nutrition of the body may be looked upon as the most important thing in our everyday life.

The term "nutrition" is a somewhat technical one, and includes all those physiological processes associated with the material growth, repair, and supplies of the body. It begins with a consideration of the composition of the body, of the source of the materials which are used in body growth and repair and the preparation of these materials, of their change within the body, and finally of the ridding of the body of waste materials. The process of nutrition is so extended and complex that it is difficult to define it in briefer terms than those just formulated above. If one were to attempt a concise definition, it would be as follows: Nutrition is the physiological process of supplying the material needs of the body.

The material needs of the body are supplied in the food and drink which we take into the stomach, and the oxygen which we inhale into the lungs. Food and drink make so important a part of the material needs of the body, and the choice and preparation of the foods are so important a part of the work of those who have the care of growing children, or of workingmen and women, or of invalids in hospitals, institutions, and homes, that the principles of such choice and preparation have been reduced to a science known as Dietetics. One would therefore define the science of dietetics as a systematic presentation of food classification and food preparation, together with the principles which govern the choice of foods under various conditions, age, employment, health, or sickness.

It must be evident that no one can gain a rational idea of nutrition and the special phase of nutrition called dietetics without a knowledge of the chemical composition of the body and of foods, also of the chemical changes which the foods undergo in the body. This knowledge of body chemistry or physiological chemistry, as it is usually called, is so essential that the early chapters of this brief work will be devoted to a concise presentation of the fundamental principles, including the most essential facts of physiological chemistry. It might be said in passing that the student of nutrition and dietetics should possess at least a general knowledge of the structure of the human body, more particularly that of the alimentary canal and the digestive system in general. Without this knowledge many of the statements of any work on nutrition and dietetics would convey only the vaguest ideas to the reader.

Furthermore, the student of dietetics should bring to the subject at least an elementary knowledge of chemistry, together with practical ideas or actual experience, in the methods of preparing and cooking foods in the kitchen.