  Ibn Daud’s Aristotelian orientation is immediately evident in his analysis of the notions of substance and accident, with which his book opens. Everything that is, is either substance or accident. Criticizing the neo-Platonic approach of Solomon ibn Gabirol, who held that substance and accident are relative terms, so that the selfsame thing can be a substance in one respect and an accident in another, he insists that substance and accident are absolute terms and that a thing is either a substance or an accident, but not both. The fact that certain substances derive their existence from a cause, and that they can be ordered hierarchically, does not detract from their substantiality: ontologically all substances are alike. This initial inquiry into the ontological constitution of existing things and the correct understanding of “substance” as that which constitutes the essence of a thing (and is thus more worthy of the term “being” than is accident) is crucial to Ibn Daud. Besides forming the point of departure for a systematic discussion of various kinds of substances, corporeal and incorporeal, it allows him to prove that the soul is a substance, not an accident. Moreover, the distinction between corporeal and incorporeal substances introduces the reader to the existence of celestial intelligences; the incorporeal simple substances that play a central role in his thought.