I know that there is no good in them, but for a man to rejoice; rather, I knew, perceived, that there was no good for them; i.e. for men. From the facts adduced, Koheleth learned this practical result—that man had nothing in his own power (see on Ecclesiastes 2:24) which would conduce to his happiness, but to make the best of life such as he finds it.
Vulgate, Cognovi quod non esset melius nisi laetari. To do good in his life; τοῦ ποιεῖν ἀγαθόν;; Facere bene (Vulgate). This has been taken by many in the sense of "doing one's self good, prospering, enjoying one's self."
like the Greek εὖ πράττειν, and therefore nearly equivalent to "rejoice" in the former part of the verse. But the expression is best taken here, as when it occurs elsewhere (e.g. Ecclesiastes 7:20), in a moral sense, and it thus teaches the great truth that virtue is essential to happiness, that to "trust in the Lord … to depart from evil, and to do good" (Psalms 36:3, 27), will bring peace and content (see in the epilogue, Ecclesiastes 12:13, Ecclesiastes 12:14).
There is no Epicureanism in this verse; the enjoyment spoken of is not licentiousness, but a happy appreciation of the innocent pleasures which the love of God offers to those who live in accordance with the laws of their higher nature.