And again, - Psalm 94:11.
The Lord knoweth - God searches the heart. The particular thing which it is here said that he knows, is, that the thoughts of man are vain. They have this quality; and this is that which the psalmist here says that God sees. The affirmation is not one respecting the omniscience of God, but with respect to what God sees of the nature of the thoughts of the wise.
The thoughts of the wise - Their plans, purposes, designs.
That they are vain - That they lack real wisdom; they are foolish; they shall not be accomplished as they expect; or be seen to have that wisdom which they now suppose they possess.

The Lord knoweth the thoughts of the wise - They are always full of schemes and plans for earthly good; and God knows that all this is vain, empty, and unsatisfactory; and will stand them in no stead when he comes to take away their souls. This is a quotation from Psalm 94:11. What is here said of the vanity of human knowledge is true of every kind of wisdom that leads not immediately to God himself.

And again,.... Not in the same place, nor in the same book, but in the Psalm, in Psalm 94:11. This form of citing Scriptures answers to and moreover, used by the Jewish doctors when the matter does not so clearly appear from the first proof, and therefore they produce another (q): and so here the apostle, for the further confirmation and illustration of this point, that the wisdom of this world is foolishness with God, to the testimony of Eliphaz, adds this of David,
the Lord knoweth the thoughts of the wise, that they are vain; in the Psalm it is, "the Lord knoweth the thoughts of men, that they are vanity"; which the apostle not only cites, but explains and teaches; as that by men is meant men of wisdom and knowledge, of the greatest capacities, whose thoughts, reasonings, schemes, and devices, the omniscient God not only knows, but makes known, and discovers them, sooner or later, to be vain and fruitless, yea, vanity itself; and notwithstanding all their machinations and contrivances, his counsel shall stand, his Gospel shall be maintained, his truths shall prevail, and his ordinances shall be continued, and his work go on.
(q) Vid. Surenhusii Biblos Katallages, p. 11, 531.

Quotation from Psalm 94:11. There it is of men; here it is "of the wise." Paul by inspiration states the class of men whose "thoughts" (or rather, "reasonings," as suits the Greek and the sense of the context) the Spirit designated in the Psalm, "vanity," namely, the "proud" (Psalm 94:2) and worldly-wise, whom God in Psalm 94:8 calls "fools," though they "boast themselves" of their wisdom in pushing their interests (Psalm 94:4).

That they are but vain - Empty, foolish; they and all their thoughts. Psalm 94:11.