Are not all the tendencies of the mind and the body accounted for by inherited aptitude? Here are two parallel lines of existence — one of the mind, the other of matter. If matter and its transformations answer for all that we have, there is no necessity for supposing the existence of a soul. But it cannot be proved that thought has been evolved out of matter, and if a philosophical monism is inevitable, spiritual monism is certainly logical and no less desirable than a materialistic monism; but neither of these is necessary here.