The Lord knoweth the thoughts of man - That is, He who teaches people all that they know Psalms 94:10, must understand all that there is in the mind. See the notes at 1 Corinthians 3:20.
That they are vanity - That is, that they are foolish, vain, unwise, wicked. The knowledge of the thoughts themselves carries with it also the knowledge that they are vain and foolish - for that is their character, and to know them truly is to know this of them. They do not appear to him as they do to people themselves. They are to his view stripped of all that is flattering and illusive, and are seen to be vain and foolish.