It is not only about the faculties that the ancient philosophers disagree. . . . They are besides in radical disagreement about the following questions: What are the parts of the soul; what is a part; what is a faculty; what difference is there between a part and a faculty ?
The Stoics divide the soul into eight parts: the five senses, speech, sex-power, and the directing (predominating) principle, which is served by the other faculties, so that the soul is composed of a faculty that commands, and faculties that obey.
In their writing about ethics, Plato and Aristotle divide the soul into three parts. This division has been adopted by the greater part of later philosophers; but these have not understood that the object of this definition was to classify and define the virtues (Plato: reason, anger and appetite; Aristotle: locomotion, appetite and understanding). Indeed, if this classification be carefully scrutinized, it will be seen that it fails to account for all the faculties of the soul; it neglects imagination, sensibility, intelligence, and the natural faculties (the generative and nutritive powers).
Other philosophers, such as Numenius, do not teach one soul in three parts, like the preceding, nor in two, such as the rational and irrational parts. They believe that we have two souls, one rational, the other irrational. Some among them attribute immortality to both of the souls; others attribute it only to the rational soul, and think that death not only suspends the exercise of the faculties that belong to the irrational soul, but even dissolves its “being” or essence. Last, there are some that believe, that by virtue of the union of the two souls, their movements are double, because each of them feels the passions of the other.