It is generally agreed that thinking should be trusted or else it should not rank very high. Popper recommended rating thinking very highly without trusting it; do not defend your views: improve upon them. Elaborations on this comprise Popper’s teaching. The whole of his output was in effort to answer critics who could not share his idea that our heritage consists of old noble errors. He tried to use his suggestion (that we should think but not trust our thoughts) as means for explaining our partiality for science and for democracy. Scientific progress is the elimination of errors; democracy is the peaceful overthrow of objectionable government. Finally, following Gottlob Frege Popper assumed that for a statement to have meaning it should be true or false; scientific theories, then, he added, mean what they say; he followed Albert Einstein and Bertrand Russell in advocating realism and in admitting that science cannot be free of metaphysical assumptions. Towards the end of his life he developed his own indeterminist and dualist metaphysics.