Ecclesiastes 2:12 (ESV)

12 So I turned to consider wisdom and madness and folly. For what can the man do who comes after the king? Only what has already been done.

The Preacher has done a thorough investigation. It has been so sound that he knows that people who investigate the same thing would come to the same conclusion if they would be honest. They will then have to admit that they cannot add anything of substance to King Solomon’s research. The king has looked into wisdom, foolishness and folly. He must come to the disappointing conclusion that everything is in vain.