Psalm 94:11

The Lord knoweth the thoughts of man
He not only hears their words, and sees their actions; but he knows their thoughts, the secret thoughts of their hearts, though he is afar off from them, and, they from him; he is the searcher of the hearts and trier of the reins of the children of men; see ( Psalms 139:2 Psalms 139:23 Psalms 139:24 ) ( Jeremiah 17:9 Jeremiah 17:10 ) , and so is Christ, who is the omniscient God, and is the Jehovah all along spoken to and of in this psalm; he knows the thoughts of men, and is a critical discerner of them, ( Matthew 9:3 Matthew 9:4 ) ( Hebrews 4:12 )

that they are vanity;
either that their thoughts are vanity; the object of them is vanity, the riches and honours of this world, which are all vanity and vexation of spirit; and sinful lusts and pleasures, which are vain and useless, yea, pernicious and hurtful: and so they are in their issue and event; they come to nothing, they are without effect; the Lord disappoints men's devices, and frustrates their designs; they think of this and the other, form schemes, but cannot execute them: or else the sense is, that they themselves are vanity, as man in his best estate is; even every man, whether of high or low degree; see ( Psalms 39:5 Psalms 39:6 ) ( 62:9 ) . The Syriac version is, "for they are a vapour"; with which compare ( James 4:14 ) . Thales, an Heathen philosopher ^F5, being asked whether men's deeds could be hid from God, answered, no, nor their thoughts.