Descartes' physics
I. Overview
1. The essence of body is extension. Consequences:
3. Motion is caused by God, who instigates it at the creation and constantly maintains it according to a principle of conservation of quantity of motion and the 3 laws of nature.
4. Conservation principle: The total quantity of motion of bodies in the universe is conserved. This is due to the immutability and constancy of God. The quantity of motion of a body is the size of the body times its speed. (Not to be identified with momentum (mass x velocity): (a) for Descartes, size and mass are not distinct concepts; (b) for Descartes, speed (scalar) and velocity (vector) are not distinct concepts.)
5. Laws of nature: They are secondary causes of motion (God is the primary cause) and are due to the immutability and constancy of God. The 3 Laws describe the ways God doles out the quantities of motion to bodies at every instant in a manner consistent with his nature (hence consistant with conservation of total quantity of motion) and the fact that there are alot of bodies in the universe (hence they collide with each other).
6. Interpretations of 3 Laws:
Garber (1992): Laws 1 and 2 are principles of persistence: they describe the motion of bodies left to themselves; Law 3 is a principle of reconciliation: it describes how the motion of bodies, as described by Laws 1 and 2, can be reconciled when they come into contact with each other.
Hatfield (1992): Laws 1 and 2 define the notion of "power" in Law 3. The power (or force) a body has to act on another or to resist the action of another is the tendancy it has to persist in its own state. This tendancy is not a property of inert matter, but an attribute of God.
7. 2 ways of conceiving of the way God interacts with bodies via the conservation principle and the 3 Laws to produce motion:
II. Excerpts from the Principles of Philosophy:
| Article 36. God is the primary cause of motion; and
he always
preserves the same quantity of motion in the universe.
After this consideration of the nature of motion, we must look at its cause. This is in fact twofold: first, there is the universal and primary cause - the general cause of all the motions of the world; and second there is the particular cause which produces in an individual piece of matter some motion which it previously lacked. Now as far as the general cause is concerned, it seems clear to me that this is no other than God himself. In the beginning, ,<in his omnipotence>, he created matter, along with its motion and rest; and now, merely by his regular concurrence, he preserves the same amount of motion and rest in the material universe as he put there in the beginning. Admittedly motion is simply a mode of the matter which is moved. But nevertheless it has a certain determinate quantity; and this, we easily understand, may be constant in the universe as a whole while varying in any given part. Thus if one part of matter moves twice as fast as another which is twice as large, we must consider that there is the same quantity of motion in each part; and if one part slows down, we must suppose that some other part of equal size speeds up by the same amount. For we understand that God’s perfection involves not only his being immutable in himself, but also his operating in a manner that is always utterly constant and immutable. Now there are some changes whose occurrence is guaranteed either by our own plain experience or by divine revelation, and either our perception or our faith shows us that these take place without any change in the creator; but apart from these we should not suppose that any other changes occur in God’s works, in case this suggests some inconstancy in God. Thus, God imparted various motions to the parts of matter when he first created them, and he now preserves all this matter in the same way, and by the same process by which he originally created it; and it follows from what we have said that this fact alone makes it most reasonable to think that God likewise always preserves the same quantity of motion in matter. (Principles Part II, art. 36) |
| From God’s immutability we can also know certain rules or laws of nature, which are the secondary and particular causes of the various motions we see in particular bodies. (Principles Part II, art. 37) |