Of things which are in our power and not in our power

Of all the faculties (except that which I shall soon mention), you will find not one which is capable of contemplating itself, and, consequently, not capable either of approving or disapproving. How far does the grammatic art possess the contemplating power? As far as forming a judgment about what is written and spoken. And how far music? As far as judging about melody. Does either of them then contemplate itself? By no means. […] What faculty then will tell you? That which contemplates both itself and all other things. And what is this faculty? The rational faculty; for this is the only faculty that we have received which examines itself, what it is, and what power it has, and what is the value of this gift, and examines all other faculties: for what else is there which tells us that golden things are beautiful, for they do not say so themselves? Evidently it is the faculty which is capable of judging of appearances.

As then it was fit to be so, that which is best of all and supreme over all is the only thing which the gods have placed in our power, the right use of appearances; but all other things they have not placed in our power. […]

Epictetus first establishes that the rational faculty — the hēgemonikon, the ruling part — is the only one that can examine itself and the phantasiai arriving at it, and so it is the only one capable of moral progress at all. The dichotomy of control — articulated more compactly in the opening of the Enchiridion ("Of things some are in our power, and others are not. In our power are opinion, movement towards a thing, desire, aversion…; not in our power are the body, property, reputation…") — is not arbitrary doctrine; it is the consequence of where the self-examining faculty actually lives.