The same arguments which induce us to believe that there is a soul in man apply to animals. The world has generally believed that in man, beside the body, there is also soul. Why have people believed it? The reason probably is, that, beside all that can be accounted for as the result of the juxtaposition of material particles, there remains a very important element unaccounted for. Mechanical and physical agency may explain much, but the most essential characteristic of vital phenomena they do not explain. They do not account for the unity in variety, permanence in change, growth from within by continuous processes, coming from the vital functions in an organized body. Every such body has a unity peculiar to itself, which cannot be considered the result of the collocation of material molecules. It is a unity which controls these molecules, arranges and rearranges them, maintains a steady activity, carries the body through the phenomena of growth, and causes the various organs to coöperate for the purposes of the whole. The vital power is not merely the result of material phenomena, but it reacts on these as a cause. Add to this that strange phenomenon of human consciousness, the sense of personality,--which is the clear perception of selfhood as a distinct unchanging unit, residing in a body all of whose parts are in perpetual flux,--and we see why the opinion of a soul has arisen. It has been assumed by the common sense of mankind that in every living body the cause of the mode of existence of each part is contained in the whole. As soon as death intervenes each part is left free to pass through changes peculiar to itself alone. Life is a power which acts from the whole upon the parts, causing them to resist chemical laws, which begin to act as soon as life departs. The unity of a living body does not result from an ingenious juxtaposition of parts, like that of a watch, for example. For the unity of a living body implies that which is called "the vital vortex," or perpetual exchange of particles.