Karl Popper, one of the great thinkers of the 20th century, is known for his influential philosophy of knowledge. Popper rejects the notion of a manifest truth, suggesting rather that our knowledge can only progress via a series of rigidly criticized – and tested – conjectures; essentially, by trial and error. In describing various self-affirming explanations – still abundant today – he wrote [stresses are Popper's]:
"Once your eyes were [..] opened you saw confirming instances everywhere: the world was full of verifications of the theory. Whatever happened always confirmed it. Thus its truth appeared manifest; and unbelievers were clearly people who did not want to see the manifest truth; who refused to see it, either because it was against their class interest, or because of their repressions which were still 'un-analysed' and crying out for treatment. [..]"
"[T]he dogmatic attitude is clearly related to the tendency to verify our laws and schemata by seeking to apply them and to confirm them, even to the point of neglecting refutations, whereas the critical attitude is one of readiness to change them – to test them; to refute them; to falsify them, if possible."
– Conjectures and Refutations, pp. 45, 66.