7. From the true idea of God follow the principles of a philosophy of nature or the doctrine of the two substances. Substance is that which so exists that it needs nothing else for its existence. In this (highest) sense God is the only substance. God, as the infinite substance, has his ground in himself, is the cause of himself. The two created substances, on the other hand, the thinking and the corporeal substance, mind and matter, are substances only in a broader sense of the word; they may be apprehended under the common conception that they are things which need only the co-operation of God for their existence. Each of these two substances has an attribute which constitutes its nature and its essence, and to which all its other determinations may be referred. The attribute and essence of matter is extension, that of mind, thought. For every thing else which can be predicated of body presupposes extension, and is only a mode of extension, as every thing we can find in mind is only a modification of thought. A substance to which thought immediately belongs is called mind, and a substance, whose immediate substratum is extension, is called body. Since thought and extension are distinct from each other, and since mind cannot only be known without the attributes of the body, but is in itself the negation of those attributes, we may say that the essence of these substances is in their reciprocal negation. Mind and body are wholly distinct, and have nothing in common. 8. We pass by the physics of Descartes, which has only a subordinate philosophical interest, and notice next his views of anthropology. From this dualistic relation between mind and matter, there follows a dualistic relation between soul and body. If matter is essentially extension, and mind essentially thought, and if the two have nothing in common, then the union of soul and body can be conceived only as a mechanical one. The body is to be regarded as an artistic automaton, which God has made, as a statue or machine formed by God from the earth. Within this body the soul dwells, closely but not internally connected with it. The union of the two is only a powerful bringing of the two together, since each is not only an independent factor, but is essentially distinct from and even opposed to the other. The body by itself is a machine fully prepared, in which nothing is changed by the entrance of the thinking soul, except that through it certain motions are secured: the wheel-work of the machine remains as it was. It is only thought which distinguishes this machine from every other; hence, therefore, brutes which are not self-conscious nor thinking, must be ranked with all other machines. From this standpoint arose especially the question concerning the seat of the soul. If body and soul are independent substances, each essentially opposed to the other, they cannot interpenetrate each other, but can touch only at one point when they are powerfully brought together.