Summary
There is one thing clearly missing for Descartes' account of motion up until this point, which is the concept of force. What gets all this motion going in the first place? Descartes takes the rest of Part II to answer this question. He divides the causes of motion into two categories. First, there is the primary and general cause, the cause that puts force into the system. Then, there are the particular causes that explain how, in each instance, a piece of matter obtains the motion that it obtains. The primary cause is God, while the particular causes are the laws of motion that he set up.
The picture is supposed to look like this: God put an initial blast of motion into the world and then set up some laws to govern the patterns of motion that objects will undergo. Since God always acts in a manner that is constant and immutable, the amount of motion in the world always remains constant. In other words, Descartes manages to derive a law of conservation of motion from God's nature.
The laws that govern motion are different from all the other principles that Descartes has presented up until this point. Since they are not properties of body but, rather, laws set up by God, they cannot be deduced from the property of extension. Instead, they have to be deduced from the immutability of the workings of God. The first law of nature is that, in the absence of friction or collision, an object in motion remains in motion, and an object at rest remains at rest. The second law is that an object in motion travels a straight path, and for an object's trajectory to curve, some further force is needed to change its path. The third, and final, law is that, when two bodies collide, the weaker body gains some motion, and the stronger body loses the corresponding amount of motion (e.g. when a projectile hits a hard body it rebounds in the opposite direction, but when it hits a soft body it stops). Principles 40–53 explain the rules that govern this transfer of motion between bodies.
Descartes ends Part II with a discussion of the difference between hard and fluid bodies. The primary observable difference between hard and fluid bodies is that fluid bodies are penetrable, while hard bodies are not. In other words, fluid bodies are quick to abandon their place to other bodies, while hard bodies are not: if you press your hand on water, the water will move away and let your hand take the place where water particles once rested; if, on the other hand, you press on a piece of concrete, the concrete will not budge. Descartes' explanation for this different behavior among bodies is that fluid bodies are made up of tiny particles which are all in motion relative to one another, while hard bodies are made up of particles that are all at rest relative to one another. If a body's parts are continually moving, then another body that comes along will be able to occupy the spaces being continually vacated by the moving particles. If a body is not moving, then no spaces are being vacated, and so no other body can find its way in.
Analysis
The picture of force, however contrived, seems, at least, easy enough to understand on a superficial level. However, there is a big worry that complicates the story. Descartes does not just think that God put force into the world at the very beginning and then left the laws of motion to do the rest. Rather, according to Descartes' picture, God constantly maintains the amount of motion in the world in the same way that he initially put it in. This is because, as we saw in Part I, Descartes believes that the world needs to be maintained by an act of divine creation at every moment. With this in mind, the picture suddenly looks like it could be very different. If God is constantly recreating the world, then he might simply position each part of matter in a different place each time he recreates it. The laws, then, would really just describe the patterns in which God recreates the world. The laws would not act to guide the force that God initially blasted in.
It is not clear which of these two pictures really describes the way Descartes imagined motion to work. The first view, on which God provides the force with the laws of motion doing the rest, is obviously much more appealing to a modern mind. The other picture hardly seems like science to us at all. There is no real indication of Descartes' opinion either way, however.
Moving on from the notion of force to the laws of motion themselves, presents another consideration. Though Descartes claims that the laws of motion can be derived from the immutability of God's operation, it is pretty clear that he could not have deduced these laws using just logical reasoning. That is to say, if he had just sat in an armchair in his home in Holland thinking about the immutability of God's operations, he could not possibly have arrived at the fact that these three laws govern all of motion. Instead, the discovery of these laws required much observation and experimentation, which, luckily, other scientists, particularly Galileo, had already done by this time. However, it is still remarkably ambitious of Descartes to attempt to provide proofs for all of these laws by referring only to the immutability of God. Unfortunately, none of these proofs is particularly convincing.
Given the ambitious scope of the project, it is worthwhile to go through each of these proofs and see how well they work. Descartes' proof for the law of inertia is this: God always works in the simplest and most immutable ways. In other words, God never allows anything to happen unless there is some good reason for it. Therefore, if something is at rest, he would never make it not at rest unless there was some reason to do this (and similarly for motion). In the absence of friction or a collision, there is no reason for particles to change their state, and so they do not. The proof for rectilinear motion is much the same: Since God operates in the simplest and most immutable way possible, he would not have the direction of motion change for no reason, and to travel without change of direction is to travel in a straight line. Descartes' proof for the third law rests on the conservation of motion. Since the amount of motion cannot change, it is just transferred between bodies. Descartes is not particularly clear on why hard bodies lose this motion and soft gain it, however.
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