Ecclesiastes 1:17

"I applied my mind to know wisdom and to know madness and folly. I perceived that this, too, is a vexation of the spirit." Contrary abstract ideas are understood by looking at contrary facts; and " wisdom is the first to be lacking in foolishness" [Horat. Epist.I, 1,41-42.], but it is not possible to be lacking in foolishness, unless one has understood it. Many dangerous things are also created from foolishness, so that while we try to avoid them, we are actually instructed in wisdom. Solomon wanted to know wisdom and knowledge with equal enthusiasm, and equally madness and folly, so that whilst seeking some things and shunning others, his true wisdom might be proved. But in this too, as in other things, he said he found great difficulties and was not able to grasp the exact truth of matters. What I have said above about "vexation of the spirit" or "suffering of the soul", as it is more often written in this book, should be sufficient to understand the rest of this verse.

Solomon is the one speaking here, the third king of Israel whom the Lord had chosen after Saul and David. He received the kingship from his father and extended his rule which brought him renown among the Israelites. Solomon no longer subjected peoples through battle; by conducting himself peacefully and with full authority, he did not devote his energy towards anything not belonging to him.… He claimed to know the efforts needed to attain pleasure and accomplished everything which he had enumerated, an experience which taught him that vanity is the common end of men's pursuits. Ecclesiastes sets forth the order in his narrative when during his youth he first had leisure for personal training, for attention to such labors does not indicate laxity. But the Spirit uses free will, a movement proper to our nature, to increase knowledge if a person is to succeed in his endeavors. Thus wisdom grows not by considering reason which closely regards passion and unreason when it comes to that deception arising from corporeal enjoyment; rather it is knowledge about vanity through experience of these endeavors.

It is Solomon who speaks these words. This Solomon was the third king of Israel, after King Saul and David, the chosen of the Lord. He succeeded his father on the throne and was proclaimed king when the power of the Israelites had already reached its height; he did not go on wearing his people out with war and fighting but lived in peace as far as lay in his power, making it his task not to acquire what did not belong to him but to enjoy what he already had in abundance.… Such is the order he adopts in his account, that first in the early years of his life he devotes his time to education and does not take the easy course in the face of the hard work such study involves but uses the choice of his spirit, that is, his natural impulse, for the accumulation of knowledge, even though his goal was achieved by hard work. And thus, when he has matured in wisdom, he does not merely theoretically observe the passionate and irrational deception of mankind in the matter of bodily enjoyments but through the actual experience of each of the things they pursue recognizes their futility.