Descartes hints that his method has not only helped him solve the many metaphysical problems he discusses in part four, but that it has also helped him make great headway in the physical sciences. He had initially written up these discoveries in a work called The World, but had suppressed its publication when he learned in 1633 that Galileo had been condemned by the Inquisition for defending the heliocentric theory toward which Descartes was also sympathetic. Rather than risk dangerous controversy by publishing this manuscript, Descartes proposes to summarize some of its results here.
The treatise initially concerns itself with the nature of light, but in the course of its discussion touches on pretty much everything Descartes has studied in nature: the sun and stars, as the source of light; the sky, as a transmitter of light; heavenly bodies, as objects that reflect light; objects on earth, that have color, are transparent, or luminous; and human beings, as perceivers of light.
In order to avoid theological controversy, Descartes says nothing about this world, but rather speaks of an imaginary world that he posits to be created by God as a random chaos of matter. God then imposes certain laws of nature that govern the behavior of this matter, and aside from that, leaves this imaginary world untouched.
Descartes's treatise first describes matter, showing it to be easily comprehensible, and shows that the laws of nature follow from God's perfection and would be the same in any world. It then explains how these laws would cause this chaos of matter to separate into mostly open space, with planets, stars, and light, just the way it is in this world. In dealing with the earth, the treatise shows how all things should tend toward its center, how there should be oceans and land, tides and rivers, plants and minerals, and how fire should work.
Descartes does not mean to say that this is how the real world was actually formed—professing that his beliefs are in accordance with holy scripture—but only that the laws of nature are such that they could have made the world the way it is out of a chaotic swirl and without God's subsequent interference.
The treatise also explains how a human body can be made to perform all its unconscious functions in accordance with these laws. Descartes is more forthcoming here, launching into a lengthy discussion largely inspired by William Harvey's discovery in 1628 of the circulation of blood through the veins and arteries. Descartes suggests further that his treatise shows how these discoveries could explain the movement of muscles, the receptivity of sensory organs, the appetites of hunger and thirst, and so on.
Nonetheless, our rational soul, which allows us to exercise reason and to speak, cannot be explained by these means, and Descartes suggests that it is a gift of God. He argues that animals have no intelligence, that this is a distinctive feature of humans. Though parrots can imitate speech, animals cannot use language to express themselves with the versatility displayed by humans. Animals have a number of amazing skills, and Descartes suggests that if these skills were a result of intelligence then they would be far more intelligent than humans are. But since they cannot speak (animals are similar enough to us that if they could speak we would understand them) they cannot have any intelligence, and their behavior is governed solely by instinct. Descartes uses these and other arguments to assert that we are distinct from animals and that we have an immortal soul that will survive our death.
Descartes wrote The World between 1629 and 1633, but then suppressed its publication in the same year that Galileo was placed under house arrest. It was subsequently published in the 1660's, after Descartes's death. As a result, we are able to judge just how far Descartes's grandiose claims are justified.
The Cartesian physics presented in The World and in his other writings are, to a large extent, the road not taken by modern physics. On the whole, this is probably a good thing, as Newtonian physics is far more accurate and detailed in many respects. Cartesian physics makes hardly any distinction between geometry and physics; both subjects deal with bodies in space, and Descartes's definition of body is broad enough not to make any clear distinction between the two. For Descartes, a body is anything that is extended in space and space is defined by the body that occupies it. For Descartes, there is no such thing as a vacuum: space is body and body is space. Thus, what we might consider to be the empty space between planets is just as much a body to Descartes as the planets themselves.
Descartes's definition of body as an extended substance is very simple and very powerful. Descartes believed that he could explain all of physics based on this very simple definition, and The World is, to a large extent, an attempt to do so. He is in fact quite successful at using this very simple first principle to explain a wide range of phenomena, but he is less successful in explaining how things change. There is no room made in his definition of body as extended substance for an explanation of force or energy. Ultimately, he can claim that there are laws of nature laid down by God that determine movement and change, but he is unable to explain the how and the why of these dynamics.
Less than a hundred years later, Newton developed his principles of physics that were based not on a definition of matter but on a definition of force. His invention (along with Leibniz's) of calculus allows him to explain how things change over time, and so allows him to explain movement and energy based on his three laws of motion. Newton's force-based mechanics, and not Descartes's matter-based mechanics, has become the norm for studying macroscopic phenomena.
We should note, however, the careful introduction of scientific method into Descartes's discoveries. He is careful to avoid Galileo's fate, and insists that he believes that God created the world according to the Bible's description in Genesis, and that his explanation is merely a thought-experiment. Rather than assert bold facts, he invents an imaginary world to show that certain laws of nature could make the world the way it is even if the story in Genesis were false. This is, in essence, the new scientific method of hypothesis and experiment.
Science since the revolution of the 16th century has been based on the idea that no theory is certain, but rather a theory is only increasingly likely as it is confirmed by more and more experiments. We have every reason to believe in Newton's law of gravitation, since everything we observe confirms it and nothing seems to contradict it. If, however, we observed some phenomena that suggested otherwise, we might need to review our theory. This in fact happened in the early twentieth century, and led to Newton's gravitation being replaced by Einstein's general relativity.
Descartes, then, is not insisting that he knows how the universe is, he is just providing a plausible hypothesis to explain how it might be. This hypothesis is in itself neither true nor false: it either fits the facts or doesn't fit the facts. To the extent to which Descartes's hypothesis fits the facts and helps us to predict and explain physical phenomena, it is a successful theory. Because medieval philosophy has a different understanding of physics—one according to which one proves and demonstrates scientific facts—Descartes's use of hypothesis and theory would seem to them to be harmless theorizing. In fact, it is an early seed of a new scientific method that will render their old method archaic within a century.
Significantly, Descartes does not try to extend his scientific theories to explain human consciousness. This is because Descartes is fundamentally a mind-body dualist. That is, he sees mind and body as two very distinct things. His physics explains the workings of bodily substances, that is, substances whose essence is extension. Mind, on the other hand is not extended. It is a substance whose essence is thought. These two different substances operate in totally different realms according to totally different laws. Explaining how mind and body can interact with one another has been a major preoccupation for philosophers ever since Descartes.