Descartes espoused mechanical philosophy, a form of natural philosophy popular in the 17th century. He thought everything physical in the universe to be made of tiny "corpuscles" of matter. Corpuscularianism is closely related to atomism. The main difference was that Descartes maintained that there could be no vacuum, and all matter was constantly swirling to prevent a void as corpuscles moved through other matter. "The World" presents a corpuscularian cosmology in which swirling vortices explain, among other phenomena, the creation of the Solar System and the circular motion of planets around the Sun.