   Farabi's familiarity with Aristotle is evident in the summary sketch of his writings that he presents in The Philosophy of Aristotle. The soul is defined as “that by which the animate substance—I mean that which admits of   life—is realized as  substance,”[12] serving the triple function of being a formal, efficient and final cause. For human beings, the intellect assumes the mantle of substance (Mahdi, 125), it being “a principle underlying the essence of man,” both an agent and final cause (Mahdi, 122). Specifically, it is the theoretical intellect that has this status, the practical intellect being subsidiary to it. The final perfection of a person is found in the actualization of this theoretical intellect, its substance being identical with its act.