The critical examination of our theories

"There is only one element of rationality in our attempts to know the world: it is the critical examination of our theories. These theories themselves are guesswork. We do not know, we only guess. If you ask me, 'How do you know?' my reply would be, I don't; I only propose a guess. If you are interested in my problem, I shall be most happy if you criticize my guess, and if you offer counterproposals, I in turn will try to criticize them.'"