Qualitative research is undergoing a surge in popularity in the medical literature, as shown by the growth in the proportion of qualitative studies in the BMJ over the past two decades (fig 1⇓). Though many qualitative studies are not obviously philosophical—for example, the authors might simply want to determine what doctors think of clinical targets—one school of thought claims that qualitative methods are derived from a constructivist philosophy. This view is widely accepted in several disciplines, including nursing.1 2 3 A recent series of papers published in the BMJ,4 5 6 7 8 9 designed to help readers to critically appraise qualitative research, devotes the whole of the first article to explaining the philosophical differences between qualitative and quantitative approaches.4 Kuper and colleagues state that the differences between qualitative and quantitative methods “run deeper than the presence and absence of numbers,” and that “most qualitative researchers today share a different belief about knowledge, called ‘constructivism,’” which is at odds with the “theoretical underpinnings” of quantitative methods.4 So what is constructivism, exactly?