Albert Barnes Commentary 1 Corinthians 3:20

"and again, The Lord knoweth the reasonings of the wise that they are vain." — 1 Corinthians 3:20 (ASV)

And again (Psalms 94:11).

The Lord knows. God searches the heart. The particular thing He is said to know here is that the thoughts of man are vain. They possess this quality, and this is what the psalmist here says God sees. The affirmation does not concern God's omniscience, but rather what God perceives concerning 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 will not be accomplished as they expect, or be seen to have the wisdom they now suppose they possess.