Aristotle, following Plato, defined the soul as the core essence of a being, but argued against it having a separate existence. For instance, if a knife had a soul, the act of cutting would be that soul, because 'cutting' is the essence of what it is to be a knife. Unlike Plato and the religious traditions, Aristotle did not consider the soul as some kind of separate, ghostly occupant of the body (just as we cannot separate the activity of cutting from the knife). As the soul, in Aristotle's view, is an
activity of the body it cannot be immortal (when a knife is destroyed, the cutting stops). More precisely, the soul is the "first activity" of a living body. This is a state, or a potential for actual, or 'second', activity. "The axe has an edge for cutting" was, for Aristotle, analogous to "humans have bodies for rational activity," and the potential for rational activity thus constituted the essence of a human soul. Aristotle used his concept of the soul in many of his works; the
Nicomachean Ethics provides a good place to start to gain more understanding of his views.
Aristotle's view appears to have some similarity to the Buddhist 'no soul' view (see below). For both there is certainly no 'separable immortal essence'. It may simply become a matter of definition, as most Buddhists would agree, surely, that one can use a knife for cutting. They might, perhaps, stress the impermanence of the knife's cutting ability, and Aristotle would probably agree with that.