Magnetism, long considered the exemplar of an occult, spiritual power, posed a challenge to mechanical philosophers like Descartes. William Gilbert's De Magnete (1600) offered an impressive natural philosophy, grounded in experiments, which could lead to interpreting magnetism as an immaterial power that possesses in its higher manifestations the capabilities of soul or mind. In his Principles of Philosophy, Descartes accepts Gilbert's experiments, but he explains magnetism mechanistically, according to the movements of two species, right – and left – handed, of “channeled” or cylindrical screw-shaped particles of his first element. Descartes claims that magnetic bodies – naturally occurring lodestone, or magnetized iron or steel – have two sets of pores running axially between their magnetic poles: one set accepts only right – handed channeled particles; the other set of pores accepts only the left – handed particles. Descartes thus explained Gilbert's experiments, including his use of a sphere of loadstone, to demonstrate the properties of magnetized compass needles.

However, Descartes did more than appropriate and reinterpret Gilbert's “laboratory” work. Gilbert called his sphere of lodestone a terrella, a “little earth,” arguing that because compass needles behave identically on the terrella as on the earth itself, the earth is, essentially, a magnet. Hence, according to his natural philosophy, the earth possesses a magnetic “soul,” capable of causing it to spin. Magnetic “souls” similarly cause the motions of other heavenly bodies. In his Principles, Descartes, aiming to displace Gilbert's natural philosophy, focuses on the “cosmic” genesis and function of his channeled magnetic particles. Descartes argues that the spaces between the spherical corpuscles of the second element that make up his vortices are roughly triangular, so that particles of the first element, constantly being forced through the interstices of second-element spheres, become “channeled” or “grooved” with triangular cross sections. Such first-element corpuscles tend to be flung by centrifugal tendency out of the equatorial regions of vortices and into neighboring vortices along the north and south directions of their axes of rotation, thus receiving opposite axial twists (see vortex). The resulting left – and right – handed screw shaped first-element particles penetrate into the polar regions of central stars and then bubble up toward their surfaces to form, Descartes claims, sunspots. Stars are thus magnetic, as Gilbert maintained, but in a mechanistic sense.

Moreover, for Descartes, planets are also magnetic, as Gilbert claimed, but again the explanation is mechanical. Descartes describes how a star may become totally encrusted by sunspots. This extinguishes the star, its vortex collapses, and it is drawn into a neighboring vortex to orbit its central star as a planet. But such planets, including our earth, bear the magnetic imprint of their stellar origins, by possessing axial channels between their magnetic poles accommodated to the right- or left-handed screw particles. Descartes’ explanation ranges from the cosmic production of magnetic particles, through the nature of stars and sunspots, to the birth and history of planets. He accepts the cosmic importance of magnetism but renders the explanation mechanical, thus binding his natural philosophy into a cosmogonical and cosmological whole.

See also Cosmology, Element, Experiment, Explanation, Principles of Philosophy, Subtle Matter, Vortex