Karl Popper and Friedrich von Hayek became close friends soon after they first met

in the early 1930s. Ever since, they discussed their ideas intensively on many occasions.

But even though an analysis of the origins and contents of their ideas and

correspondence reveals a number of important and fundamental differences, they rarely

criticize each other in their published work. The article analyzes in particular the

different ideas they have on the role of reason in society and on rationalism and the

roots of these differences. Popper’s "Towards a Rational Theory of

Tradition" of 1948 contains a criticism of Hayek’s idea—published,

for instance, in "Individualism: True and False" of 1945—that we

must accept tradition without trying to change it. An analysis of the differences

between the two authors touches on topics such as the possibility of public intervention

in society, the role of social science in this, the methodology of social science, and the

differences between liberalism and social democracy. The article concludes with some

possible explanations for Popper and Hayek downplaying their differences in public. The

fact remains that they never resolved the tension between Popper’s critical

rationalism and Hayek’s conservative rationalism.