Both the philosopher Karl Popper and the physicist Philipp Frank had close personal connections with Vienna and were responsive to the political forces that impinged on its cultural life. By the late 1930s both had left central Europe and both had engaged with the topic of relativism. Both found the theme of relativism to be important for their work on scientific methodology and for exploring the relation of scientific knowledge to its social context. David Bloor uncovers the traces of the relativism debates.

Despite these similarities, Popper and Frank operated with very different definitions of relativism. Frank identified himself as a relativist but, according to Popper’s definition, Frank was not a relativist. Conversely, Popper identified himself as an anti-relativist but, according to Frank’s definition of relativism, Popper ought to have been a relativist. The aim of this lecture will be to analyse the respective definitions that are at work here and explain how this, potentially confusing, situation could arise. But after the necessary work of exposition and clarification, David Bloor will address two further themes. First: which of their competing definitions is the better? Second: what in this debate is relevant to the contemporary problems of today? Popper and Frank wrote in an historical setting that was overshadowed by World War II and the Cold War. Though today’s discussions of relativism take place in a different context, there are still threats to the rational conduct of science and politics. Was Popper right to see relativism as one such threat, or was Frank right when he saw relativism as a means to defend science and rationality?