=The Relation of God to Matter.= Descartes here investigates the realm in which he has the deepest interest; but he makes a concession at the very beginning. He divests things of their qualities and finds the essence of matter to be extension. Qualities are not resident in things, but are the result of our sensations. Sense-perception is knowledge of qualities, and therefore obscure knowledge; while clear or intellectual knowledge is of quantities. But there is one quality common to matter,――extension. Space, extension, and matter are the same. There is no space that is empty, no matter that is not extended. An extended or material body has, however, in itself no principle of motion. It cannot move itself. It must be moved by an external cause, and the whole universe must be a mechanism whose movements have their first cause in God. Matter in its modes of motion and rest has God as its first cause or unmoved mover; and under matter is included everything extended,――inanimate objects, the lower animals, and the bodies of men. To this world of matter God stands in the relation of an inventor to his machine. =The Relation of God to Minds.= The essential nature of minds is thought. Mind is therefore different from matter because it is unextended and free. The two relative substances have nothing in common except that they are related to God. The relation of God to minds is, however, very different from His relation to matter. God is not the unmoved mover of minds, but He is the perfect and infinite mind to which our finite minds turn as their ideal. God thinks and wills perfectly what we think and will imperfectly. He is not the mechanical but the teleological cause of minds, their _ens perfectissimum_, the goal of all mental aspiration. =The Relation of Mind and Body.= In proportion as Descartes clearly defined mind and body, and referred each back to its own principle, the impossibility of connecting the two became apparent. Descartes intended that his theory should, above everything else, clear philosophy of all obscurities. So he divided the world into two relative substances,――mind and matter,――each operating in its own realm, each exclusive of the other. The intention of Descartes is to be a consistent dualist. But there was one point where, with one eye on the church, he had to qualify for ethical considerations his scientific principle of matter. That is the point where the human body acts upon the soul and the soul acts upon the body.