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.