Having surveyed prior opinions about the soul, Aristotle is now ready to state his own views. Here he offers an “outline definition and sketch of the soul” (158).

Soul is a substance and is the form of the body. Soul is immaterial; body is material. Matter by its nature is potential; it must be informed by form to become a particular thing—a body, a chair, or whatever. Thus, soul is the actuality of the body; it gives the body life. Soul is what determines the nature of a living being. For example, if the eye were itself a body, then sight would be its “soul” (158). Soul is then the substance and form of a living thing. Moreover, the soul is not separable from the body; indeed, it is the living being. Rather than saying a human being has a soul, it would be truer to say that he is his soul.

Aristotle emphasizes that a good definition of something shows not only “what it is” but also “the reason for being as it is” (159). This is what he hopes to do in his inquiry into the soul. Aristotle affirms that soul is the life-principle of living things; it is what makes them alive.