The explicit theme of this article is the question of knowledge in education and, more particularly, the importance of the differentiation of knowledge as a starting point for the sociology of the curriculum. It also has two subthemes: change and difference. First, the author argues that if the sociology of education is to make its potential contribution to curriculum debates, it must "change" its approach to knowledge. Second, he contrasts the sociology of the curriculum of the 1970s and 1980s, which opposed an emphasis on the specificity of curricula differences as masking wider social differences, with the position taken in this article, that certain knowledge "differences" need defending on both educational and sociological grounds. It can be argued that he is documenting little more than a familiar story in the cyclical history of ideas, when one extreme, social constructivism, is replaced by another, social realism. His response to this challenge is twofold. First, this article is frankly autobiographical and he is assuming that his experience of searching for a more reliable sociology of the curriculum is unlikely to be unique and that a self-conscious recognition of past mistakes may have some value for others struggling with similar issues. Second, the debates between what he has referred to as social constructivism and social realism have a long history and indeed have a partly cyclical character. (Contains 30 notes.)