This paper provides a critical review of the debate over the philosophical foundations of mixed methods research and examines the notion of philosophical foundations. It distinguishes axiology-oriented from ontology-oriented philosophical foundations. It also identifies three different senses of philosophical foundations of mixed methods research. The weak sense of philosophical foundations (e.g., pragmatism) merely allows the possibility of the integration of both quantitative and qualitative methods/data/designs. The moderate sense of philosophical foundations (e.g., transformativism) provide a good reason to use mixed methods (...) in (at least some) social scientific research. The strong sense of philosophical foundations (e.g., dialectical pluralism) justifies a normative thesis that mixed methods research should be encouraged in (at least some) social scientific research. (shrink)