The power of Freewill is obvious but unspeakably difficult to explain.

And if he is unwilling to be convinced in this way, it is because the power of his Freewill is so great, and our mouth is unable to do it full justice ; our weak mouth has confessed that it is unable to state its unconstrained Will. For it is a Freewill which subjects even God to Investigation and rebuke, on account of its unconstrained nature. It ventured to bring up all this because it desired to speak about that which is unspeakable. [Ov. p. 41, l. 5.] But that (Freewill) which has ventured to make statements concerning God, itself is not able to state its own nature perfectly. But concerning this, also, we say to any one who asks that this is a marvel which it is very easy for us to perceive, but it is very difficult to give a proof of it. [But it is impossible to explain anything completely.] But this is not so only in this matter, but it is the same with everything. For whatever exists may be discussed without being searched out; it can be known that the thing exists, but it is not possible to search out how it exists. For see that we can perceive [xvi]  everything, but we cannot completely search out anything at all ; and we perceive great things, but we cannot search out perfectly even worthless things. [Let us thank God that our Knowledge of things is limited.] But thanks be to Him Who has allowed us to know the external side of things in order that we may learn how we excel, but He has not allowed us to know their (inward) secret that we might understand how we are lacking. He has allowed us, therefore, to know and not to know that by means of what can be known, our childish nature might be educated, and that our boldness might be restrained by those things which cannot be known. Therefore, He has not permitted us to know, not that we may be ignorant, but that our Ignorance may be a hedge for our Knowledge. [Knowing that our powers of knowing are so limited we can avoid vain and weary searching.] For see how we wish to know even the height of heaven and the breadth of the earth, but we cannot know ; and because we cannot know we are thus restrained from toiling. Therefore, our Ignorance is found to be a boundary for our Knowledge, and our want of Knowledge (lit. simpleness) continually controls the impetuosity of our boldness. For when a man knows that he cannot measure a spring of water, by the very fact that he cannot, he is prevented from drawing out what is inexhaustible. [Ov. p. 42, l. 5.] And by this experience it is seen that our weakness is a wall in the face of our boldness. Thus, too, when we know that we cannot know, we cease to investigate. For if, when we know little, the impetuosity of boldness carries us on and proceeds to those things which may not be known, who is there who will not give thanks to Him. Who has restrained us from this wearisomeness, even if we do not wish to remain within the just boundary within which He has set us ? Our Ignorance, therefore, is a bridle to our Knowledge. [Yet we are not to be ignorant, but to seek after practical Knowledge.] And from these instances it does not follow that the All-knower wished to make us ignorant, but He placed our Knowledge under a helpful guardian ; and better is the small Knowledge which knows the small range of Ignorance than the great Knowledge which has not recognized its limits ; and better is the weak man who carries about something that is necessary for his life than the arrogant strong man who burdens himself with great stones which cause his destruction. [Our chief Knowledge is to know what subjects can never be known.] But our chief Knowledge is (just) this—to know that we do not know [xvii]  anything. For if we know that we do not know, then we conquer Error by our Knowledge. For when we know that everything that exists is either known or not known, thereby we acquire the true Knowledge. For whoever thinks he can know everything, falls short of the Knowledge of everything. For by means of his Knowledge he has gained for himself Ignorance. But whoever knows that he cannot know, from Ignorance Knowledge accrues to such a one. [Ov. p. 43.] For in virtue of the fact that he knows that he cannot know, he is enabled to know, that is to say, (he knows) something which profits him.