It is only in the nineteenth century that our knowledge of human life has attained the dignity of a genuine, independent science; during the course of the century it has developed into one of the highest, most interesting, and most important branches of knowledge. This "science of the vital functions," physiology, had, it is true, been regarded at a much earlier date as a desirable, if not a necessary, condition of success in medical treatment, and had been constantly associated with anatomy, the science of the structure of the body. But it was only much later, and much more slowly, than the latter that it could be thoroughly studied, as it had to contend with much more serious difficulties. The idea of life, as the opposite of death, naturally became the subject of speculation at a very early age. In the living man, just as in other living animals, there were certain peculiar changes, especially movements, which were wanting in lifeless nature: spontaneous locomotion, the beat of the heart, the drawing of the breath, speech, and so forth. But the discrimination of such "organic movements" from similar phenomena in inorganic bodies was by no means easy, and was frequently impossible; the flowing stream, the flickering flame, the rushing wind, the falling rock, seemed to man to exhibit the same movements. It was quite natural that primitive man should attribute an independent life to these "dead" bodies. He knew no more of the real sources of movement in the one case than in the other. We find the earliest scientific observations on the nature of man's vital functions (as well as on his structure) in the Greek natural philosophers and physicians of the sixth and fifth centuries before Christ. The best collection of the physiological facts which were known at that time is to be found in the _Natural History_ of Aristotle; a great number of his assertions were probably taken from Democritus and Hippocrates. The school of the latter had already made attempts to explain the mystery; it postulated as the ultimate source of life in man and the beasts a volatile "spirit of life" (Pneuma); and Erasistratus (280 B.C.) already drew a distinction between the lower and the higher "spirit of life," the _pneuma zoticon_ in the heart and the _pneuma psychicon_ in the brain.