This paper starts from a problem posed by matter in Aristotle’s investigation of substance in Metaphysics Zeta and Eta 1-5, and then argues that Aristotle solves the problem in Eta 6 and Theta 7 as part of a two-pronged solution to the larger question: What is primary substance? Both prongs use the notions of potentiality and actuality, and the second prong in Theta 6 and 8, building on the first, allows Aristotle to argue that a subset of hylomorphic complexes—living organisms—are primary substances