In his chapter, Clancey develops his transactional perspective by taking an interactional viewpoint as his primary point of reference. In this chapter, I argue that the distinction that Clancey draws between the interactional and transactional perspectives is a difference that makes a difference when theorizing about learning and practice. I first draw on Dewey's pragmatism to clarify the criterion of usefulness as explanatory power that I use to assess the potential contributions of Clancey's transactional perspective to the collective enterprise of mathematics education. I then focus on specific aspects of Clancey's transactional perspective to illustrate its relevance in orienting instructional design and the analysis of learning in classroom settings.