But extended substance, the kingdom of nature, and spiritual substance, do not require one another. They may be called substances, because each of them constitutes an entire range or sphere, an independent totality. But because, Spinoza concluded, each side, the kingdom of thought as well as nature, is one complete system within itself, they are likewise in themselves, that is absolutely, identical as God, the absolute substance; for thinking spirit this implicit is thus God, or their differences are ideal.
Descartes proceeds from the Notion of God to what is created, to thought and extension, and from this to the particular. “Now substances have several attributes without which they cannot be thought" - that signifies their determinateness - "but each has something peculiar to itself which constitutes its nature and essence" - a simple universal determinateness - "and to which the others all relate. Thus thought, constitutes the absolute attribute of mind,” thought is its quality;“extension is" the essential determination of corporeality, and this alone is “the true nature of body. What remains are merely secondary qualities, modes, like figure and movement in what is extended, imagination, feeling and will in thinking; they may be taken away or thought away. God is the uncreated, thinking substance.” (30)Descartes here passes to what is individual, and because he follows up extension he arrives at matter, rest, movement. One of Descartes’ main points is that matter, extension, corporeality, are quite the same thing for thought; according to him the nature of body is fulfilled in its extension, and this should be accepted as the only essential fact respecting the corporeal world. We say that body offers resistance, has smell, taste, colour, transparency, hardness, &c., since without these we can have no body. All these further determinations respecting what is extended, such as size, rest, movement, and inertia, are, however, merely sensuous, and this Descartes showed, as it had long before this been shown by the Sceptics. Undoubtedly that is the abstract Notion or pure essence, but to body or to pure existence, there likewise of necessity pertains negativity or diversity. By means of the following illustration Descartes showed that with the exception of extension, all corporeal determinations may be annihilated, and that none can be absolutely predicated. We draw conclusions respecting the solidity and hardness of matter from the resistance which a body offers to our disturbance, and by means of which it seeks to hold its place. Now if we admit that matter as we touch it always gives way to as like space, we should have no reason for ascribing to it solidity. Smell, colour, taste, are in the same way sensuous qualities merely; but what we clearly perceive is alone true. If a body is ground into small parts, it gives way, and yet it does not lose its nature; resistance is thus not essential.(31) This not-being-for-itself is however a quantitatively slighter resistance only; the resistance always remains. But Descartes desires only to think; now he does not think resistance, colour, &c., but apprehends them by the senses only. Hence he says that all this must be led back to extension as being special modifications of the same. It is undoubtedly to the credit of Descartes that he only accepts as true what is thought; but the abrogation of these sensuous qualities simply represents the negative movement of thought: the essence of body is conditioned through this thought, that is, it is not true essence.
Descartes now makes his way from the Notion of extension to the laws of motion, as the universal knowledge of the corporeal in its implicitude; he shows that there is no vacuum, for that would be an extension without bodily substance, i.e. a body without body; that there are no atoms (no indivisible independent existence), for the same reason, viz., because the essence of body is extension. He further shows that a body is set in motion by something outside of it, but of itself it continues in a condition of rest, and likewise it must, when in a condition of movement, be brought to rest by another outside of it - this is the property of inertia.(32) These are unmeaning propositions, for an abstraction is involved for instance in asserting simple rest and movement in their opposition.
Extension and motion are the fundamental conceptions in mechanical physics; they represent the truth of the corporeal world. It is thus that ideality comes before the mind of Descartes, and he is far elevated above the reality of the sensuous qualities, although he does not reach so far as to the separation of this ideality. He thus remains at the point of view of mechanism pure and simple.
Give me matter (extension) and motion and I will build worlds for you, is what Descartes virtually says.(33) Space and time were hence to him the only determinations of the material universe. In this, then, lies the mechanical fashion of viewing nature, or the natural philosophy of Descartes is seen to be purely mechanical.(34) Hence changes in matter are due merely to motion, so that Descartes traces every relationship to the rest and movement of particles, and all material diversity such as colour, and taste - in short, all bodily qualities and animal phenomena - to mechanism. In living beings processes such as that of digestion are mechanical effects which have as principles, rest and movement. We here see the ground and origin of the mechanical philosophy; but further on we find that this is unsatisfactory, for matter and motion do not suffice to explain life. Yet the great matter in all this is that thought goes forward in its determinations, and that it constitutes from these thought-determinations the truth of nature.
