Albert Barnes Commentary Ecclesiastes 8:14

"There is a vanity which is done upon the earth, that there are righteous men unto whom it happeneth according to the work of the wicked; again, there are wicked men to whom it happeneth according to the work of the righteous: I said that this also is vanity." — Ecclesiastes 8:14 (ASV)

Which is done upon the earth - The instance of vanity, to which these words are applied, is the seeming inequality of God’s justice; but if they are considered in connection with the profession of personal faith in God’s absolute justice (Ecclesiastes 8:12), the conclusion is irresistible, that, whatever reason the Preacher had for reserve in declaring his belief, he certainly looked forward to final judgment in a future state of existence (Ecclesiastes 12:14).