Aristotle’s main thesis in Metaphysics Z.17, which takes substance to be a principle and a cause of some sort (1041a9–10, 1041b7–9, b30–31), is of a piece with the assumption that hylomorphic compounds are unified wholes (1041b11–12) – an assumption that proves critical to settling an important controversy about the form-matter relationship in that chapter, i. e. whether matter and form are mutually indistinguishable or rather just accidentally the same. By rejecting these interpretive options, this paper argues that form and matter (...) are bound together by an essential link which, nevertheless, is able to preserve both (i) the different explanatory roles that each of those principles play vis-à-vis specific substances and (ii) the compound’s proper unity. (shrink)