Wise men study the sciences which deal with the origins and development of animal life, with the structure of the cells, with the effect of various diseases upon the tissues and fluids of the body; they study the causes of the reactions of the body cells to disease germs, and search for the origin and means of extermination of these enemies to health. They study the laws of physical well-being. They seek for the chemical principles governing the reactions of digestive fluids to the foods they must transform into heat and energy. So the doctor learns to combat disease with science, and at the same time to apply scientific laws of health that he may fortify the human body against the invasion of harmful germs. Thus, eventually, he makes medicine itself less necessary. But another science must walk hand in hand today with that of medicine; for doctors and nurses are realizing as never before the power of mind over body, and the hopelessness of trying to cure the one without considering the other. Hence psychology has come into her own as a recognized science of the mind, just as biology, histology, chemistry, pathology, and medicine are recognized sciences governing the body. As these are concerned with the "how" and "why" of life, and of the body reactions, so psychology is concerned with the "how" and "why" of conduct and of thinking.