According to most scholars, in the Parts of Animals Aristotle frequently provides explanations in terms of material necessity, as well as explanations in terms of that-for-the-sake-of-which, i.e., final causes. In this paper, I argue that we misunderstand both matter and the way that Aristotle explains things using necessity if we interpret Aristotle as explaining things in terms of material necessity. Aristotle does not use the term “matter” very frequently in his detailed discussions of animal parts; when he does use it, he typically identifies blood as matter. I argue that this is because blood is, for Aristotle, what properly nourishes and grows the other parts of the body and he views nourishment and growth as types of coming-to-be. Th...