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 Psa 139:2, 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, Mat 9:3. 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 Psa 39:5. The Syriac version is, "for they are a vapour"; with which compare Jam 4:14. Thales, an Heathen philosopher (e), being asked whether men's deeds could be hid from God, answered, no, nor their thoughts. (e) Apud Laert. Vit. ejus, Valer. Maxim. l. 7. c. 2. extern. 8.