Aristotle’s argument for the nature of self relied on hylomorphism or the questions of material, formal, efficient, and final.
According to Aristotle, the human soul has the form of its ‘vessel’ (the body); the soul is one’s real self. However, the soul is not composite and thus not subject to change. Then, a living body is matter (‘hylo’), and a soul is a form; it makes the body matter into a living thing (‘morphe’).
Avicenna may object to this argument by asserting the existence of self “without also having [to] assert the existence of any of his exterior or interior parts” can happen. Thus, one may argue that a soul and a body can be separable, and the presence of a body is not needed to be self. The question of the soul’s function in the body is answered.
To such an objection, one can respond that the soul being separable or inseparable from the body is not the primary factor in changing the nature of the self. Arguably, separate or not, the interaction between the form and the matter nonetheless matters the most.