God only is wise; and by his answer he intends to show that the wisdom of men is worth little or nothing
 Bertrand Russell, A History of Western Philosophy (1945). copy citation

Context

““Then I knew that not by wisdom do poets write poetry, but by a sort of genius and inspiration.” Then he went to the artisans, but found them equally disappointing. In the process, he says, he made many dangerous enemies. Finally he concluded that “God only is wise; and by his answer he intends to show that the wisdom of men is worth little or nothing; he is not speaking of Socrates, he is only using my name by way of illustration, as if he said, He, O men, is the wisest, who, like Socrates, knows that his wisdom is in truth worth nothing.” This business of showing up pretenders to wisdom takes up all his time, and has left him in utter poverty, but he feels it a duty to vindicate the oracle.” source