By contrast, in the past four decades, an increasing appreciation of his critique has helped us to better understand the phenomenal growth of scientific theory and the close relationship between science and the humanities. In his books Objective Knowledge and Conjectures and Refutations, Popper demonstrates brilliantly the roles of myth and metaphysics in the scientific enterprise. Metaphysics is the work we do when we carry out comparative analysis of our cosmological myths and theories. If there are priests of myth who insist on perpetuating the myths without correction or revision, there are others among us who both subject the myths to criticism and offer rival theoretical explanations. Far from being meaningless, critical metaphysics and cosmology provide the cognitive background for the growth of scientific theory. In some ways, science is the metaphysics that succeeded in spawning bold theories which are not only well articulated and critically debated but also observably testableand by testable, Popper means falsifiable. Perhaps the major contribution made to science by Popper emerges from his argument that the job of scientific experiment is to seek evidence not to support a proposed theory but, rather, to refute it. According to Popper, the whole point of seeking to shoot down our scientific theories is not simply to increase our supply of skepticism. Poppers humanism shines brightest when he urges us to seek out criticism of our theories. Instead of advocating that we pile up sufficient positive evidence to prove or verify a belief, Popper offers an entire new way to think about testing our beliefs and corroborating them. Furthermore, Poppers epistemology makes no fetish of either skepticism or faith. Trust and faith, like skepticism, are essential ingredients to human living. The beauty of Poppers evolutionary theory of knowledge lies in its insistence that imagination and speculation are essential ingredients of the thinking process. If Poppers analysis is correct, then both evolution and creationism are theories. Contrary to what some creationists claim, scientists tend to favor evolution as an explanatory theory not because of some presupposition that blinds them to the truth but, rather, because it is scientifically more fruitful than creationism and enjoys greater explanatory power. But according to Poppers epistemology, since a presupposition is only a conjecture or conclusion used to help spawn other conjectures, there is no reason why presuppositions cannot be debated and criticized. According to Popper, objectivity is, therefore, not a psychological state of mind purified of all biases and presuppositions (he never confuses an open mind with a blank mind) but, rather, a two-pronged openness: openness to severe criticism and openness to look into alternative or even rival theories. Poppers theory of learning not only allows for indoctrination but requires it. Indeed, some humanists in the past seem to have believed that objectivity or openness of mind required weak indoctrination. From the perspective of Karl Popper, by contrast, indoctrination should be thorough, not in the sense of shutting off all criticisms but in the sense of being done competently and by someone who is informed and articulate. Indoctrination moves toward education only as it is combined with openness to criticism and to rival indoctrinations, views, conjecture, theories, and doctrines.
The difference between science and non-science is that a scientific theory can be tested and falsified. Using this idea, Popper contrasts the science of Einstein and the pseudo-science of Freud.
He also spent a year studying philosophy at Yale University on a post-graduate fellowship.