THE WORLD.--Thus sure of himself, of God, and of the world, Descartes studies the world and himself. In the world he sees souls and matter; matter is substance in extensions, souls are substance not in extension, spiritual substance. The extended substance is endowed with impulse. Is the impulse self-generated, are the bodies self-impelled? No, they are moved. What is the primary motive force? It is God. Souls are substances without extension and motive forces. In this respect they are analogous to God. They are united to bodies and act on them. How? This is an impenetrable mystery, but they are closely and substantially united to the bodies, which is proved by physical pains depressing the soul and moral sufferings depressing the body; and they act on them, not by creating movements, for the quantity of movements is always the same, but by directing the movements after this fashion or that. Souls being spiritual, there is no reason for their disaggregation, that is, their demise, and in fact they do not die. It is for this reason that Descartes lays such stress on animals not having souls. If they had souls, the souls would be spiritual, they would not be susceptible to disaggregation and would be immortal. "Save atheism, there is no doctrine more dangerous and detestable than that," but animals are soulless and purely mechanism; Descartes exerts himself to prove this in great detail, and he thus escapes avowing the immortality of the souls of animals, which is repugnant to him, or by allowing that they perish with the bodies to be exposed to the objection: "Will it not be the same with the souls of men?"