Although problem-based learning is being adopted by many institutions around the world as an effective model of learning in higher education, there is a surprising lack of critique in the problem-based learning literature in relation to its philosophical characteristics. This paper explores epistemology as a starting point for investigating the theoretical underpinnings of problem-based learning as a learning model. Criticisms of empiricism are analysed in terms of the perceived learning outcomes of learners undertaking a problem-based learning curriculum. It is argued that models of empiricism theorised by philosophers such as Bacon, Locke and Hume cannot fully account for the learning model found in problem-based learning curricula. It is proposed that an alternative epistemological approach is needed. The work of Karl Popper is discussed, whose critical rationalist epistemology emphasises the generation of bold conjectures and criticism. Popper's work shows a positive contribution to the demands of higher education, characterised by learners who are serious about making professional progress. The paper concludes by critically analysing the tensions and contradictions of problem-based learning in light of Popper's epistemological theory of critical rationalism. It is argued that a critical rationalist perspective has educational benefits for students as it creates an environment rich in critical thinking, reading and writing and values disjunction and challenge. (A bibliography is included. Contains 1 table.)