It is our plan, so far as possible, to discuss the problems of the mind in one section of this book, and the problems of the body in another; but just as we found that we could not separate our duties to ourself from our duties to our neighbors, so we find that the mind and the body are inextricably interwoven, and that whenever we probe deeply into one, we discover the other. The interaction of the mind and the body is a fascinating problem into which we must look for a moment, not because we expect to solve it, but because it illuminates the whole subject. The human body is a machine. It takes in carbon and oxygen, and burns them, and gives out carbon dioxide and other waste products, and develops energy in proportion to the amount of carbon it consumes. This machine has its elaborate apparatus of action and reaction, its sensory organs where outside stimuli are received, its nerves like telegraph wires to carry these impressions, its brain cells to store them and to transform them into reactions. We know to some extent how these brain cells work. We know what portions of the brain are devoted to this or that activity. We know that if we stick a pin into a certain spot we shall paralyze the left forefinger. We know that by injecting a certain drug, or by breathing a certain gas, we can cause this or that sensation or reaction, such as laughing or weeping or mania. We know what poisons are generated in the system by anger, and what chemical changes take place in a muscle that is tired. All this is part of a vast new science which is called bio-chemistry, or the chemistry of life.