 Just as the motion of a ball can be affected by the bodies it encounters, so too can light be affected by the bodies it encounters. Depending on how these bodies are themselves physically constituted, they either reflect or refract light. Soft bodies, such as a linen sheets, sand, or mud “completely stop the ball and check its movement”, while hard bodies simply “send the ball in another direction without stopping it” (AT 7: 89, CSM 1: 155). Since the tendency to motion obeys the same laws as motion itself, Descartes demonstrates the law of refraction by comparing refracted light to the motion of a tennis ball before and after it punctures a linen sheet 