 In Chapter 1 of APO (“On the Nature of Light”), Kepler expounds 38 propositions concerning different properties of light: light flows in all directions from every point of a body’s surface; it has no matter, weight, or resistance. Following—but also inverting—the Aristotelian argument for the temporality of motion, he affirms that the motion of light takes place not in time but in an instant (in momento). Light is propagated by straight lines (rays), which are not light itself but its motion. It is important to note that although light travels from one body to another, it is not a body but a two-dimensional entity which tends to expand to a curved surface. The two-dimensionality of light is probably the main reason why it is incorporeal. Motion in general plays a significant role in Kepler’s philosophy of light. For Straker, the supposed link between optics and physics (especially in Prop. 20, where the mechanical analyses are introduced) “reveals the full extent of his commitment to a mechanical physics of light” (Straker 1971, p. 509).