Animals, in Aristotle's view, are paradigm instances of substance-being. We may wonder whether Aristotle began with that conviction and shaped his ontology in the light of it, or arrived at it as a result of what his ontology revealed the nature of substance to be, and that question in turn may be related to our views about when and with what attitude he did his biological work. But whatever we decide to say on these questions, our answers will have to take account of his clear conviction that natural living things are ‘above all substances’.

We should therefore expect that Aristotle thought the most important features of substance to be exemplified in the case of living entities. This need not mean that he claimed that only animals are substances. On the whole, Aristotle was less concerned with the correct identification of a class of entities which are substances than with the proper understanding of the principles and modes of being by virtue of which those entities which we commonly understood to be substantial beings are substantial. He was, we might say, less interested in substances than in substance-being, less concerned with the question of what beings are substances than with the question of what it is to be a substance. But among the beings that Aristotle saw as commonly (and therefore in some sense correctly) understood to exemplify substance-being, animals came first, and those features which metaphysics reveals as characteristic of substance-being should therefore be evident in the being of animals.