In the encyclopedic treatise, the ash-Shifā’ (“The Healing”), the most prominent philosopher of classical Islam Ibn Sina (Avicenna) (980–1037) has substantially revised the Aristotelian teaching about the soul. In the first instance, this revision is evident in terms of orientation to a strictly apodictic epistemology and to a synthesis of the Aristotelian teaching with Islamic monotheism. Following this second trend, Ibn Sina supplies the psychology with new dimensions – cosmological, angelological, prophetological and eschatological (more precisely, soteriological). The first book of the “Psychology” (an-Nafs) mainly deals with the notion of the soul. Ibn Sina comments on the traditional peripatetic definition of the soul with relation to the doctrine of the souls of the celestial spheres. In this context is highlighted the operational rather than the essential nature of this qualification and is also given its other modification. While analyzing the three classical characteristics of the soul (i.e. “potency”, “form” and “entelechy” – the realization of potentia), the philosopher justifies the “entelechy” as the most adequate term for it.