As a result of his alarm at the outcome of Galileo's trial, Descartes decided not to publish the manuscript of Le monde he had planned to offer as a New Year's gift to his friend Marin Mersenne at the start of 1634. Though not yet finished at the end of 1633, the publication we now know under the English title as The World, or Treatise on Light, reads as a fully developed text in itself. Le monde is written in a style much lighter than either the Rules for the Direction of the Mind or Descartes’ other early texts and echoes the thrill and enthusiasm with which its author had developed a revolutionary line of thought after initially having embarked on the explanation of a set of meteorological phenomena in 1629.

In the spring of that year, the German Jesuit Christoph Scheiner (1573–1650) had made some precise observations of the occurrence of no less than five parhelia, or sundogs, at Frascati, near Rome. Through Nicolas-Claude Fabri de Peiresc (1580–1637) and Pierre Gassendi (1592–1655), Scheiner's description was handed down to Henri Reneri (1593–1639) (see Reneri, Henricus), the Walloon professor of philosophy whom Descartes had followed to Deventer and Utrecht. In October, Descartes decided not only to explain this phenomenon but to write a full treatise on meteorology. Within a month, he had abandoned this plan, too, and was preparing a complete physics on all natural phenomena. As Descartes would later explain in the Discourse on Method (AT VI 41–42, CSM I 132), his account of the nature of light served him as a central theme to which other subjects could be related. For the original treatise, he developed not only the subjects we now find in The World, such as the origin of heavenly bodies from elementary particles, their orbits, and the explanation of gravity, but also topics that he would later incorporate in the text of the Principles of Philosophy, such as his explanation of the tides.

Having abandoned his former conviction that knowledge is somehow structured along the lines of geometrical intuition and imagination, Descartes, on his return to the Netherlands, nevertheless stuck to another line of argument from the Rules: the idea that true knowledge is based on the use of “simple natures” on account of which different things may be related to each other.