We understand Aristotle’s soul–body hylomorphism better if we first understand the critical discussions of his predecessors which occupy most of the first book of his De Anima.  Given that he regards his view as preferable to all earlier approaches, he must also think that his alternative, hylomorphism, avoids the pitfalls he identifies in those positions. In some cases, it is easy to see why he might think hylomorphism is defensible where they are not: for instance, he regards the reductively materialistic views of the earliest natural philosophers as explanatorily impoverished. In other cases, however, this is far from clear. Aristotle highlights for special consideration the view that the soul is a harmonia (attunement) of the body, a view which, as was noted in antiquity, bears more than a passing resemblance to his own hylomorphism. It proves both difficult and instructive to determine, then, how he supposes hylomorphism avoids the problems he identifies in the doctrine of the soul as a harmonia. The core difference, it emerges, turns on Aristotle’s thoroughgoing teleology: the soul, he thinks, unlike a harmonia, has an intrinsic good toward which the body is orchestrated.

I examine the status of Aristotle’s science of soul and argue that it is trans-generic in the way that Aristotle's universal mathematics is. For just as the branches of the latter differ considerably, so too do the sciences of life: botany, zoology, psychology, and (in Aristotle’s view) astronomy and theology. Discovering the correct definition of soul, which is their starting point or first principle, as with other scientific starting points, involves both induction and dialectic. Induction uses scientific observation of living things to move toward this starting point. Dialectic enables the scientist to assemble endoxa, or reputable beliefs, that allow us to solve each puzzle (aporia) that clouds our understanding (nous) of the starting point that induction enables us to reach.

In DA I.2–5, Aristotle offers a series of critical discussions of earlier Greek definitions of the soul. The status of these discussions and the role they play in the justification of Aristotle’s theory of soul in DA II–III is controversial. In contrast to a common view, I argue that these discussions are not dialectical but philosophical. I also contend that Aristotle does not consider earlier philosophical definitions of soul to be endoxa, but rather contradoxa – beliefs about which the many and the wise disagree among themselves. Through an analysis of Plato’s and Empedocles’s definitions of soul, I show that these definitions are nevertheless treated by Aristotle as potential scientific principles for explaining two of the soul’s per se attributes: causing motion and cognition in animate bodies. The main role of the critical discussions in DA I.2–5 is to show that all such earlier definitions of soul fail this explanatory task. Nevertheless, I show that these chapters are not wholly aporetic. Aristotle makes progress by solving two scientific puzzles within them: whether the soul has spatial parts, and whether ‘soul’ refers to a uniform entity across biological species.