Culturally relevant pedagogy (CRP) described in Ladson-Billings' framework has three conceptions: conceptions of self and others, conceptions of knowledge, and conceptions of social relations. Instructors can support students with positive learning experiences when they are cognizant of all three conceptions. In 13 interviews of physics instructors about CRP, we found many instructors shared only two productive conceptions: conceptions of self and others and conceptions of social relations. However, we found two instructors who effectively described all three conceptions. Their conceptions of knowledge focused on physics knowledge as constructed, reconstructed, and in some aspects containing subjectivity. They both articulated a critical approach to examining the historical and contextual factors that influence physics learning today. Our findings indicate that instructors' beliefs about the nature of science can be entangled with their own enactment of CRP and we show how a sophisticated epistemology can be one route to more culturally relevant instruction.