When you want to know what something is worth, who do you ask? Who gives you the answer? How can any other ability be stronger than this one — the one that uses all the others like servants and tests each one and makes judgments about them? Which of your abilities knows what it actually is and what it's worth? Which one knows when to work and when to stop? What ability opens and closes your eyes? What turns them away from things they shouldn't look at and points them toward other things? Is it your eyesight? No. It's your will. What ability opens and closes your ears? What makes them curious and eager to listen, or completely uninterested in what's being said? Is it your hearing? No. It's nothing other than your will.
And if you inquire what is the value of each thing, of whom do you inquire? who answers you? How then can any other faculty be more powerful than this, which uses the rest as ministers and itself proves each and pronounces about them? for which of them knows what itself is, and what is its own value? which of them knows when it ought to employ itself and when not? what faculty is it which opens and closes the eyes, and turns them away from objects to which it ought not to apply them and does apply them to other objects? Is it the faculty of vision? No, but it is the faculty of the will. What is that faculty which closes and opens the ears? what is that by which they are curious and inquisitive, or on the contrary unmoved by what is said? is it the faculty of hearing? It is no other than the faculty of the will.