The Will is its own explanation.

And if any one ask, what then is this Will ? we must tell him that the real truth about it is that it is the power of Free-choice. And because it is not right to scorn a good learner, let us now like those who hasten and pass on throw him a word, that is to say, one of the words of Truth. For, even from a single word of Truth, great faith dawns in a sound and wise hearer ; just as a great flame is produced by a small coal. For if a single one of a few coals of fire is sufficient to make scars on the body, one of the words of Truth, also, is not too weak to clean away the plague spots of Error from the soul. If, therefore, any one asks, "What is this Will, for though it is one thing, part of it is good, and part of it evil ?" we shall tell him that it is because it is a Will. And if he asks again, we shall tell him that it is a thing endowed with independence. And if he still continues to indulge in folly, we shall tell him that it is Freewill. And if he is not convinced this unteachableness of his teaches that because there is Free-will he does not wish to be taught. But if he is convinced when they say to him that there is no Freewill, it is truly wonderful that in the annulling of his Freewill, his Freewill is proved, that is to say, by his being in a desperate state. [The very denial of Freewill proves that it exists.] And the matter is as if some eloquent person wished to harangue and to prove that men have no power of Speech. And that is great madness; [xiv] for he says there is no power of Speech when he uses the power of Speech. For his power of Speech refutes him, for by means [Ov. p. 39.] of Speech he seeks to prove that there is no power of Speech. When Freewill, too, has gone to hide itself in a discussion and to show by argument that it does not exist, then is it with more certainty caught and seen to exist. For if there were no Freewill, there would be no controversy and no persuasion. But if Freewill becomes more evident when it hides itself, and when it denies (its own existence) it is the more refuted, then when it shows itself it is made as clear as the sun.