This essay aims to analyze the structure of Aristotle's Metaphysics Θ by explicating various senses of the term δύναµις at issue in the treatise. It is argued that Aristotle's central innovation, the sense of δύναµις most useful to his project in the treatise, is the kind of capacity characteristic of the pre-existent matter for substance. It is neither potentiality as a mode of being, as recent studies maintain, nor capacity for `complete' activity. It is argued further that, in starting with (...) the κύριος sense of δύναµις as capacity for change, Aristotle begins with the most familiar and acknowledged kind of capacity, in order to move to the less familiar but ultimately more useful notion of capacity for substance, and to bring these two kinds together through an analogical relation. (shrink)