 Kilwardby’s lengthiest discussion on sense perception is found in the work De spiritu fantastico. Here, Kilwardby argues that for an act of perception to occur, certain requirements need to be met: there needs to be an object, a sense organ, and a medium between them; the object must issue/generate a likeness (image or species) of itself by means of which it can be known (since nothing in the world can be known except by generating a species of itself). The likeness must be able to change the medium and then the sense organ. Both the medium and the sense organ, in different ways, must be appropriate to the likeness, that is, must be receptive of it. In the case of sight, for instance, the medium must be transparent, and there must be light (E 5, 42).