Prediger 2:19

"And who knoweth whether he shall be wise or foolish? and he will have power over all my labour with which had wearied myself, and had acted wisely, under the sun: this also is vain." או...ה, instead of אם...ה, in the double question, as at Job 16:3. What kind of a man he will be no one can previously know, and yet this person will have free control (cf. שׁלט, p.641) over all the labour that the testator has wisely gained by labour - a hendiadys, for חכם with the obj. accus. is only in such a connection possible: "my labour which I, acting wisely, gained by labour." In view of this doubtful future of that which was with pains and wisely gained by him, his spirit sank within him.

Solomon having pronounced all vanity, and particularly knowledge and learning, which he was so far from giving himself joy of that he found the increase of it did but increase his sorrow, in this chapter he goes on to show what reason he has to be tired of this world, and with what little reason most men are fond of it. I. He shows that there is no true happiness and satisfaction to be had in mirth and pleasure, and the delights of sense (Ecc 2:1-11). II. He reconsiders the pretensions of wisdom, and allows it to be excellent and useful, and yet sees it clogged with such diminutions of its worth that it proves insufficient to make a man happy (Ecc 2:12-16). III. He enquires how far the business and wealth of this world will go towards making men happy, and concludes, from his own experience, that, to those who set their hearts upon it, "it is vanity and vexation of spirit," (Ecc 2:17-23), and that, if there be any good in it, it is only to those that sit loose to it (Ecc 2:24-26).

2:19 Foolish, destructive successors are more of a threat to wise accomplishments than wise successors, but since the Teacher couldn’t tell what kind of successors he would have, his accomplishments were meaningless to him. Solomon’s immediate successor—his son Rehoboam—turned out to be foolish (see 1 Kgs 12:1-20).