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After posing his model of planetary motion, Descartes next moves on to attempt an account (purely theoretical, he is careful to mention) of how the observable universe came into being. Let us assume, Descartes tells us, that originally all the matter of the universe was divided (by God, of course) into particles of moderate and roughly equal size. Let us also suppose that God had already put in the fixed amount of motion, which has remained constant to this day. Finally, let us suppose that these particles moved in two motions: First, they moved individually around their own centers, thus creating the fluid heavens. (Remember, a fluid is simply a body in which all the particles are quickly moving in relation to one another.) Second, they moved as groups around certain centers, thereby creating vortices in the heavenly fluid.
From this motion of moderately sized particles, the entire visible universe arose. The particles congregated through their motion into three sorts of matter that still exist today and are the three elements. The first element is matter made up of particles moving so quickly that whenever this matter comes into contact with another body, it shatters into tiny component particles. The sun and the stars are composed of this element. The matter of the second element is made up of spherical particles, which are microscopic, but have a stable, fixed quantity, unlike the particles in the first element. This element forms the heavens and accounts for the nature of light. Finally, there is the third element, out of which the planets and comets are formed. The particles of this element are larger and not well-suited to motion, and so they give rise to hard bodies.
Using these three elements, together with the laws of motion and principles of physics from Part II, Descartes is able to give (rather oddly and unsatisfyingly) explanations of all observable heavenly phenomena. He tackles, among other topics, the nature of light (explained as the result of particles of the second element moving in rectilinear motion away from the center of their vortices), the spherical shape of the sun and stars (similarly explained in terms of the rectilinear motion of particles away from the center of vortices), sun spots (explained with reference to the motion of the highly agitated particles of the first element), solidity (accounted for by the size and shape of the particles of the third element), and why the motion of the planets does not describe perfect circles (because of the oddities of the continuous circuit of motion necessitated by the plenum).
Before launching into his account of the universe's origins, Descartes makes clear that he does not believe that this process of development really took place. Since God is perfect, he asserts, he would not have created the world in anything but its perfect, finished form. Why, then, does he even give this long-winded account? The answer is up for some debate.
One plausible explanation is that Descartes was being insincere in this statement. Though he claims that he believes God created the universe fully formed, it is highly likely that this is yet another instance of Descartes hedging his bets for fear of Church condemnation. The Bible tells us that God created the world fully formed, and so Descartes does not want to contradict this claim. Yet, he does want to give an account of how he thinks the universe was really formed. Making this declaration of innocence is his way of playing it safe.
However, Descartes himself was a highly religious man, and it is possible that he truly did believe in the scientific authority of the Bible. In that case, there is another explanation available for the presence of this discourse. In principle III.46 Descartes states, "For although we know for sure that they never did arise in this way, we shall be able to provide a much better explanation of their nature by this method than if we merely described them as they are now." By accounting for how these objects might have come into being, we can get a more thorough understanding of their nature, Descartes claims here.
This does not seem quite right, though. We certainly can get a much better grasp of A's nature by attempting to account for how A really might have come into being, but by merely telling a fictional story about how A could-have-but-did-not-really come into being, we do not learn anything new. All that we get out, in that case, is what we put in: the facts about A's nature that we already knew and used in order to develop a plausible fictional account of A's origin. Once we arrive at the plausible fiction, though, we do not learn anything new. We just learn that we have come up with an account that logically coheres with the facts we already knew. It is only if we then further suppose that this account might be true that we have potentially learned something new about A. Using this new knowledge (however tentative) we can deduce further hypotheses about A's nature. In the absence of any level of commitment to the truth of this account, however, the account itself is a dead end.
Of course, it is possible that Descartes only intended to lead us to this dead end. There is still some use to a pure fiction, even if it does not lead us to new knowledge. In order to come up with the story at all, we need to take stock of all the facts we already know about A's nature. Coming up with the fiction, then, can be seen as a useful way to focus us on all the facts we already know about A. Perhaps Descartes' purpose is only this. It seems, though, that he has higher hopes pinned on his account of the origin of the universe. It sounds—both from his statement of purpose and from the account itself—that he believes that he is arriving at new knowledge concerning the nature of the universe. If this is the case, then he must certainly believe that this account is more than just a useful fiction. He must believe that it is a plausible candidate for the truth. Even more than that, given his confidence in his clear and distinct perceptions, he probably believes that it is the truth.