The biology of to-day distinguishes living from inanimate bodies by the possession and exercise of the three principal properties or functions of metabolism, irritability, and reproduction; and further, the body which performs these functions is not only composed of chemically complex substances--proteids--which are not found in things that are not alive, but possesses a structure. In no case, even the simplest, is the organism a mere homogeneous lump of protoplasm, but it has parts or organs, visibly different from one another, and obviously correlated with the activities appropriated to each; and it is the preservation of that structure, in the individual and in the race, which is the end towards which the collective performance of all these functions, or the life of the organism, is apparently directed. Some of these peculiarities are shared by certain things that are not commonly regarded as alive. Crystals have of course a definite structure; they can divide, and when broken they can make good the missing part, but they do not assimilate to the substance of their own bodies a food-material which is less complex than it, and they are not irritable. The differences, indeed, between the living and the lifeless are so profound, that it is not to be wondered at that there should have been in all ages natural philosophers who have held that living activities are phenomena _sui generis_, differing _toto caelo_ from the properties exhibited by lifeless bodies, and never by any conceivability to be expressed in terms of these. This doctrine is vitalism. It exists in several varieties, but one at least is of very ancient lineage and can be traced back through mediaeval times to the biological speculations of the Greeks. Whether Aristotle really held the vitalistic views which have since been attributed to him is a matter we shall have to discuss later on, but it is certain that in the writings of Galen there is to be found a theory of life which bears the stamp of Aristotelian influence, and was destined to hand that influence on to future generations. Galen admits the sensitive soul of Aristotle as the peculiarity of animals, and the rational soul for man, but substitutes for the nutritive soul certain works of nature--attraction, repulsion, retention, alteration. And further, the rational soul is no longer immortal, but perishable, and is dependent on the body, where its seat is in the brain; it is material or quasi-material, a πνευμα, most efficient when dry. After a long interval this doctrine reappears in the sixteenth century in the writings of Vesalius, who tells us that the heart has a vital soul, the liver a natural soul, while there is elaborated in the ventricles of the brain an animal spirit or principal soul.