Aristotle's inquiry into the definitional question “what is substance?” in the central books of the Metaphysics is constrained by the unity requirement. Roughly, a particular hylomorphic compound substance, such as this human, ought to be a unified whole and not just a heap of material parts and form. A similar claim applies to the substance‐kind, human, which Metaphysics ΖΗΘ characterises as a hylomorphic compound taken universally. I raise the following question about this picture of unity: Is a compound's unity basic (...) or is it derived from the form's unity? Reading closely Metaphysics Z.12 and H.6, I argue that the form is explanatorily basic for the compound's unity. (shrink)