 The change in medium is different from that in the sense organ, and different species seem to move (in) the medium in different ways. Kilwardby connects this with the materiality and even the different degrees of materiality of the species: the species of smell and hearing move with the airwaves, whereas the species of color do  not.[11]  With respect to sight—the prototypical sense discussed in medieval theories of perception—the object, according to the rules of optics, must stand opposite the perceiver’s eyes, and the species move along a straight line. There are two objects of sight: light and color; but whereas color (and by means of it the colored thing) is that which is perceived, light is that which makes this perception possible (QLIII2S 5, 18). Light is not perceived as such; only the mind (not the sensory soul) is able to distinguish light from color, by reflection (QLIII2S 26, 190).