Karl Popper (1902-1994) was an early (proto)-antijustificationist philosopher from Austria who spent much of his career in Oxford, England (Karl Popper, being of Jewish descent, had to leave Austria when it was annected by Nazi Germany in 1938). Although he had a Marxist period in his teens, he later rejected its deterministic view of history (at the same time as he rejected Freud's and Adler's psychological theories). At that point, he created the falsifiability criterium, that it must be possible to disprove a theory in order for it to be scientific. Karl Popper criticized the empirism/rationalism debate for being a false dichotomy (see the empirism/rationalism false dichotomy. His theory of active consciousness rejected both the empirist idea of a "blank slate" and the rationalist idea of a specific set of innate ideas. Instead, Karl Popper advocated the theory that brains guess wildly and randomly without limits and that those inner guesses are later pruned by falsification when experience shows that they do not pan out (as shown in brain, that idea is well supported and important to explain how science can exist at all). He also used his active consciousness model as evidence for the existence of a physical world and against solipsism and strong idealism.

But of course even Karl Popper did not know everything. In his book "The open society and his enemies" he claimed that punishing intolerant individuals was sometimes necessary to defend an open society. But then, Karl Popper retired before punishment was shown to be counterproductive (see moderating the free will debate) and died before tolerant environments were shown to create extreme recoveries after brain damage unexplainable by established neurological and psychological theories, see the "Mind, Brain and Education" metastudy in brain, (meaning that intolerance should really be fought by splitting intolerant groups instead of punishing intolerant individuals).

Karl Popper called his own philosophy critical rationalism.

This is an article about scientific method.