Aristotle asserts that inquiry into the nature of the soul is “among the first kinds of knowledge” because it is “the first principle of living things” (126) and thus helps us understand nature and truth as a whole. However, it is difficult to find the proper method for investigating the soul. This is because one must first distinguish a number of things about the soul, such as whether it is a substance unto itself or a quality that inheres in something else, whether it has parts, whether it is actual (something that acts) or potential (something that is acted upon), whether all souls are the same, whether animal and human souls are different, and many other such things.

As a starting point, Aristotle declares that the affections of the soul (e.g., anger, fear) are inseparable from the body in the sense that they depend on the body for their operation. Moreover, one can analyze things like the affections by considering either their material operation or the rationale behind them. For example, anger can be defined as boiling of the blood or as the desire for revenge (129). Aristotle questions which of these two lines of inquiry belongs specifically to the philosopher.