Karl Popper’s Debt to Leonard Nelson

Karl Popper has often been cast as one of the most solitary figures of twentieth-century philosophy. The received image is of a thinker who developed his scientific philosophy virtually alone and in opposition to a crowd of brilliant members of the Vienna Circle. This 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. The motive for doing so is the conviction that the mainstream perspective on Popper’s philosophy makes him more difficult to understand than might otherwise be the case. The thinker who figures most significantly in the account of Popper developed in these pages is Leonard Nelson. Both a neo-Friesian and neo-Kantian, this philosopher deeply influenced Popper through his student Julius Kraft, who met with Popper on numerous occasions in the mid 1920s. It is in the light of this influence 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”.