  According to William, souls are among the simplest of created substances. Nevertheless, like all created substances, they must involve some kind of composition. This composition cannot be a bodily composition, of course, since they are not bodies and thus cannot be divided into bodily parts. According to Avicebron, as well as some of William's contemporaries, souls are composite in the sense that they are composites of form and matter, though not a matter that involves physical dimensions, but rather what some called “spiritual matter.” This view is part of the doctrine now termed “universal hylomorphism,” according to which every created substance is a composite of form and matter (see Weisheipl 1979). William is one of the first to reject this doctrine; he is later followed by Aquinas. In support of his rejection, in On the Universe he makes one of his rare references to Averroes, citing with approval his claim that prime matter is a potentiality only of perceptible substances and hence is not found in spiritual substances. He also argues that there is no need to posit matter in angels or souls in order to explain their receptivity in cognition, as some had thought necessary (OO I, 851b-852b). Rather, souls and other spiritual substances are “pure immaterial forms” without matter.