Pluralism is the philosophical position that the ultimate reality of the universe consists of a plurality of entities; it is an ontological position. But the concept of pluralism can be applied at a variety of levels: to the (epistemological) understanding of reality (whether its ultimate nature is a plurality or not), to the methods employed to theorise about that understanding of reality, to the methodology which sets the criteria for theory choice and to the study of methodologies themselves. Pluralism has been advocated at all of these levels in economics discourse. Yet an understanding of what is entailed by methodological pluralism and pluralism of method has been hampered by lack of reference to epistemological and ontological foundations. In particular, pluralism takes on a different meaning in a closed-system mode of thought (as in mainstream economics) from its meaning in an open-system mode of thought (as in Post-Keynesian economics or institutional/evolutionary economics). The former can be thought of as ‘pure pluralism’, as the dual of a monist position, while the latter involves a more limited, although crucial, pluralism.