Aristotle claims that a sensible substance is composed of form and matter, while he insists that it is a unity in a strict sense. So there is the question—in what sense can a composite thing be a unity? Aristotle’s key solution lies in his account of matter as potentiality and form as actuality. Many scholars are bewildered by his laconic solutions, and there are mainly two approaches undertaken in interpretations. One is called “projective”; the other is called “explanatory.” But neither interpretation is satisfying. The main tasks of this paper are to reexamine the problem and the two interpretations, then to argue that the composition of a sensible substance should be understood in light of its coming-to-be; that its unity refers to its being a functional unity.