The notion of probability received great attention from 20th-century mathematicians and philosophers alike. This chapter focusses on a number of thinkers who not only devoted great efforts to the notion of probability and its foundations, but also developed a thoroughly probabilistic epistemological perspective. Special attention will be paid to Hans Reichenbach, Harold Jeffreys, and Bruno de Finetti. Although these authors embraced diverging interpretations of probability, namely frequentism in the case of Reichenbach, logicism in the case of Jeffreys, and subjectivism in the case of de Finetti, they shared the conviction that probability is an essential ingredient not just of science, but of human knowledge at large, and laid the foundations of a probabilistic approach to epistemology that is today mainstream.