This leads us to a more focussed discussion of the notion of substance, ousia, and also the role in Galen’s explanations of powers or capacities (dunameis). The notion of ousia in Galen is a complex one, because he uses the term both to refer to material stuffs of various kinds (as in the everyday contemporary English usage of the term “substance”), and in an Aristotelian metaphysical sense, to refer to something’s essence, what it defintionally is (see above, §3.1). Thus, ousia is used as synonymous with matter (hulē), and contrasted with form, in the embryological treatise Sem.; but the question, “what is the ousia of the soul?” admits of non-materialist as well as materialist answers.