It is obvious that the same thing will never do or suffer opposites in the same respect in relation to the same thing and at the same time. So that if ever we find these contradictions in the functions of the mind we shall know that it was not the same thing functioning but a plurality.
Now on the subject of psychology some of the teaching current in extraneous discourses is satisfactory, and may be adopted here: namely that the soul consists of two parts, one irrational and the other capable of reason. (Whether these two parts are really distinct in the sense that the parts of the body or of any other divisible whole are distinct, or whether though distinguishable in thought as two they are inseparable in reality, like the convex and concave sides of a curve, is a question of no importance for the matter in hand.)
Republic IV argues from inner conflict, such as thirst held back by reason, to really distinct parts of the soul, and ends with three: reason, spirit and appetite. Aristotle adopts a plainer division into reason and the irrational from current teaching he does not attribute, and sets aside, as unimportant for ethics, whether the parts are separable or only distinct in thought. He then divides the irrational part again so that desire can heed reason as a child heeds a father, which grounds his split between intellectual and moral virtue. That the borrowed teaching is Platonic is a common reading; the text does not say so.
