Grazer Philosophische Studien 86 (2012), 137–156. KARL POPPER’S DEBT TO

Karl Popper has often been cast as one of the most solitary fi gures of twentieth-century philosophy. Th e received image is of a thinker who developed his scien-tifi c philosophy virtually alone and in opposition to a crowd of brilliant members of the Vienna Circle. Th is paper challenges the received view and undertakes to correctly situate on the map of the history of philosophy Popper’s contribution, in particular, his renowned fallibilist theory of knowledge. Th e motive for doing so is the conviction that the mainstream perspective on Popper’s philosophy makes him more diffi cult to understand than might otherwise be the case. Th e thinker who fi gures most signifi cantly in the account of Popper devel-oped in these pages is Leonard Nelson. Both a neo-Friesian and neo-Kantian, this philosopher deeply infl uenced Popper through his student Julius Kraft, who met with Popper on numerous occasions in the mid 1920s. It is in the light of this infl uence that we understand Popper’s recollection that when he criticized the Vienna Circle in the early 1930s, he looked upon himself “as an unorthodox Kantian”