A brutish man knoweth not - A man who is stupid, and who is like the beasts or brutes; that is, a man whose tastes and propensities are like the brutes, or who does not seem to act as if endowed with a rational nature. The idea evidently is, that there are many such people, and that it is not to be wondered at that they have no exalted idea of the greatness of God. As a matter of fact there are many in human form - many made in the image of God - who seem to have no more notion of God, and who see no more wisdom and goodness in his works, than the horse or the ox. Compare Isaiah 1:3.
Neither doth a fool understand this - A fool, in the sense that he has been made foolish and stupid by sin; that he does not worship and honor God. He has no right understanding in regard to the Maker and the Governor of the universe.