Saint Augustine On Free Will (De Libero Arbitrio), II, 32-37, 39.
32 Augustine. Number seems of little value to men, and wisdom precious, because they can count numbers more easily than they can acquire wisdom. Do not be surprised at this, for you see that men regard gold as more precious than the light of a lamp, though it is absurd to value gold in comparison. But they honor more highly a thing much lower because even a beggar lights his lamp, and only a few have gold. I do not suggest for a moment that wisdom is found lower when compared to number, since it is the same; but it demands an eye capable of discerning it. Light and heat are perceived, fused together, so to speak, from one fire, and cannot be separated from each other; yet heat is communicated to what is put near the fire, while light is diffused far and wide. So too the power of understanding which wisdom contains, heats what is closer to it, such as a rational soul, but does not affect what is more distant, such as bodily things, with the warmth of wisdom; it only shines on them with the light of number. Perhaps this is obscure to you, but no analogy from a visible thing can be made applicable in every respect to what is invisible. Only notice this point, which is sufficient for our problem and is apparent even to more lowly minds such as our own. Although we cannot be clear whether number resides in wisdom or is derived from wisdom, or whether wisdom itself is derived from number or resides in number, or whether both terms can be shown to refer to the same thing, yet it is certainly plain that both are true, and true unchangeably.
33 Therefore you would by no means deny that there exists unchangeable truth, containing all those things which are unchangeably true. You could not call this yours or mine or any man's, but it is present and offers itself in common to all who behold unchangeable truths, like a light which in a wonderful fashion is both secret and public. No one could say that anything which is present in common to all who have reason and understanding belongs to the nature of one individual. You remember, I think, our discussion a little while ago about the bodily senses. We decided that the common objects of the sense of sight or of hearing colours and sounds, for instance, which you and I both see at the same time do not share the nature of our eyes or ears, but are common objects of perception. So you would certainly not say that what you and I perceive in common, each with his own mind, shares the nature of the mind of either of us. You could not say that what the eyes of two people see at the same time is the eyes of either of them; it is something else to which both of them direct their sight.
Evodius. That is manifestly true.
34 Augustine. Do you think that this truth, about which we have been talking for such a long time, and in which, though one, we see so many things, is higher than our minds, or equal to them, or lower? If it were lower, we should make judgments about it, not in accordance with it. We make judgments about bodily things because they are lower; we often say not only that such and such is true of them, but also that it ought to be. Similarly, not only do we know that our souls are in a particular state, but often that they ought to be. And in the same way we judge about bodily things, and say, It is not so bright as it ought to be, or not so square and so on; and of souls, that is not so ready as it ought to be or not so gentle or not so vigorous according to the nature of our character. In making these judgments we follow the principles of truth within us, which we see in common. No one ever makes these the object of a judgment. When a man says that the eternal is superior to the temporal, or that seven and three are ten, no one asserts that it ought to be so, but, knowing it is so, we rejoice to make the discovery without scrutinizing and trying to correct it. If this truth were on an equality with our minds, it would itself be subject to change. Sometimes our minds see it more clearly, sometimes less clearly, and as a result they admit themselves to be subject to change. The truth, however, abiding in itself, gains nothing when we see it more clearly, and loses nothing when we see it less clearly, but, whole and sound, it gladdens with its light those who are turned towards it, and punishes with blindness those who are turned away from it. Again, we judge about our own minds according to the truth, though we can by no means judge about the truth itself. We say, 'our mind understands less than it ought,' or, 'it understands as much as it ought.' But the mind ought to understand more in proportion as it approaches, and clings to, the unchangeable truth. Hence if the truth is neither inferior to nor equal to our minds, it can only be higher and more noble.
35 I had promised, you may remember, to show you something higher than our mind and reason. This thing is truth itself. Embrace it if you can, and enjoy it; and “delight in the Lord, and He will give thee the requests of thy heart.” What more do you ask than to be happy? What is happier than the man who enjoys the firm, unchangeable, most excellent truth? Men declare they are happy when they embrace the fair bodies, ardently desired, of wives and even of harlots, and can we doubt of our happiness in the embrace of truth? Men declare they are happy when with parched throats they reach an abundant and healthful spring of water, or when they are hungry and discover a dinner or supper, richly furnished. Shall we deny our happiness when we are given the food and drink of truth? We often hear men declare they are happy if they lie amid roses and other flowers, or enjoy the sweet smell of ointments. What is more fragrant, what more delightful, than the inspiration of truth? Do we hesitate to call ourselves happy, when so inspired? Many place their lives' happiness in song, in the music of lyre and flute: when these are missing, they count themselves wretched; when these are present, they are transported with joy. When the truth, tuneful and eloquent in its silence, falls noiselessly, as it were, upon our minds, shall we seek elsewhere for a happy life, and not enjoy that which is so sure and so near at hand? Men take delight in gleaming gold and silver, in glittering gems and colors, in the light itself which our eyes perceive in fire upon the earth, or in the stars, the moon, or the sun; men take delight in the splendor and graciousness of these things. When neither poverty nor trouble keeps them from such enjoyment, they count themselves happy and for these things they wish to live forever. Are we afraid to set the happiness of life in the light of truth?
36 Since the supreme good is known and grasped in the truth, and since that truth is wisdom, let us see in wisdom the supreme good, and grasp and enjoy it. The man who enjoys the supreme good is indeed happy. The truth shows men all the things which are truly good, and each man, understanding these according to his capacity, chooses for his enjoyment one or several of them. Among those who choose an object to look at in the light of the sun and who take pleasure in the sight, some may possess strong, healthy, vigorous eyes, and these men are perfectly ready to gaze at the sun itself, which also illuminates other objects in which weaker eyes take pleasure. So too a strong, vigorous, mental gaze, when it sees with certainty many unchangeable truths, turns to the truth itself in which all things are shown; to this it clings as though forgetful of all else, and in it enjoys all things together. For whatever is delightful in other truths, owes its delightfulness to the truth itself.
37 Our freedom consists in submission to the truth, and it is our God Himself who frees us from death, that is, from the state of sin. For truth itself, speaking as a man with men, says to those who believe in Him: “you continue in my word, you shall be my disciples, indeed, and you shall know the truth, and the truth shall make you free.” The soul enjoys nothing with freedom unless it enjoys it securely. No one, however, possesses securely those goods, which he can lose against his will. But no one loses truth and wisdom against his will, for no one can be separated from them physically. That which we call separation from truth and wisdom is a perverted will, which loves lower things. No one wishes for something against his will. We have, therefore, in the truth a possession which we can all enjoy equally and in common; there is nothing wanting or defective in it. It receives all its lovers without stirring their envy; it welcomes all, and is chaste with each. One man does not say to another: go back and let me come; take away your hands and let me embrace it. All cling to it; all touch it at the same time. It is a food which is never divided; you drink nothing from it which I cannot drink. When you share in it, you make nothing your private possession; what you take from it still remains whole for me too. I do not wait until you surrender the inspiration it gives you before I can be inspired; no one ever takes any part of it for his private use, but it is wholly common to all at the same time.
39 If I showed there was something above our minds, you admitted you would confess it to be God, provided there was nothing else higher. I accepted your admission, and said it was enough that I should show this. For if there is anything more excellent, it is this which is God, but, if there is nothing more excellent, then truth itself is God. Whichever is the fact, you cannot deny that God exists, and this was the question we set ourselves to debate. If you are influenced by what we have received on faith through the most holy teaching of Christ, namely, that there is a Father of Wisdom, remember that we have also received this on faith that Wisdom, begotten of the eternal Father, is His equal. We must ask no further questions about this, but hold it firmly by faith. God exists, and He exists truly and supremely. We not only hold this, I think, by our faith as certain, but we also attain to it by a sure, though very feeble, kind of knowledge. This suffices for the question we have undertaken, and enables us to explain the other matters connected with it. Or have you any objections to raise?
Evodius. I accept this with a joy past belief, which I cannot express to you in words. I declare it to be most certain. My inner voice declares this, and I desire to be heard by the truth itself, and to cling to it. This I grant to be not only good, but the supreme good, and the source of happiness.
N.B. This text is one of the Augustine texts in the 1925 Philosophy Reader (Philosophisches Lesebuch) by Joseph Ratzinger's professor Gottlieb Söhngen. The above translation is here.