In Chapter 1, I propose a solution to Aristotle’s puzzle about what method we should follow in order to discover the essential definition of soul. I call this the method of scientific inquiry. I argue that Aristotle casts earlier Greek psychologists as engaged in trying to explain the soul’s basic per se attributes, and that for this reason, he views their theories as possible subjects of scientific criticism. To carry out this criticism, I argue, Aristotle advocates using a simple test by which to judge definitions of an entity under investigation, which I call the demonstrative heuristic. This heuristic consists in using the tools of dialectic and deduction, as well as a critical form of imagination, to test the consequences of adopting competing definitions of an item whose nature is under investigation.