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 me