Descartes’ corpuscular-mechanical natural philosophy is intended to replace the Aristotelianism of the late medieval universities and the resurgent Neoplatonic natural philosophies, in which light is conceived as the intermediary between base matter and higher spiritual and immaterial entities. In the simplest version of his theory, Descartes explains light mechanically as a tendency to motion, an impulse, propagated instantaneously through continuous optical media. This has the very important implication that in Descartes’ theory the propagation of light is instantaneous, but the magnitude of the force conveyed by the tendency to motion constituting light can vary – there can be stronger and weaker light rays, all propagated instantly (Schuster 2000, 261).

Descartes’ theory of light cannot be understood in detail without his theory of corpuscular dynamics (see force and determination). Descartes holds that bodies in motion, or tending to motion, are characterized from moment to moment by the possession of two sorts of dynamical quantity: the absolute quantity of the “force of motion”; and the directional modes of that quantity of force, which Descartes calls “determinations.” As corpuscles undergo instantaneous collisions, their quantities of force of motion and determinations alter according to the laws of nature. Descartes focuses on instantaneous tendencies to motion, rather than finite translations in space and time. His exemplar for applying these concepts is the dynamics of a stone rotated in a sling (Figure 13) (AT XI 45–46, 85; G 30, 54–55).

Descartes considers the stone at the instant that it passes point A. The instantaneously exerted force of motion of the stone is directed along the tangent AG. If the stone were released and nothing affected its trajectory, it would move along ACG at a uniform speed reflecting its conserved quantity of force of motion. However, the sling continuously constrains what can be termed the “principal” determination of the stone and, acting over time, deflects its motion along the circle AF. The other component of determination acts along AE, completely opposed by the sling, so that only a tendency to centrifugal motion occurs rather than centrifugal translation. It is this conception of centrifugal tendency that Descartes uses when he articulates his theory of light inside his cosmological theory of vortices.