In this theoretical paper, I review the history of research in educational psychology that inspired Shulman's notion of pedagogical content knowledge (PCK) and I critically examine interpretations of PCK reflected in prominent theoretical frameworks for mathematical knowledge for teaching (MKT). I propose a theory of PCK--grounded in radical constructivism and Piaget's genetic epistemology--that addresses limitations of these prominent frameworks. I conclude with a description of what makes PCK in the proposed theory "pedagogical" and describe a research agenda that reconnects MKT scholarship to its Deweyan philosophical foundations.