Teacher-teacher dialogue is a central activity within many professional learning programs. Understanding how and why dialogue works as an effective tool for teacher change is a question, however, that needs more careful probing in the extant literature. In this paper, I draw upon the philosophical theory of practical reason in order to show why and how teacher-teacher dialogue plays such a crucial role in the evolution of teacher practice. This conceptual analysis also builds our understanding as to the kinds of teacher-teacher dialogues that are more likely to be effective at catalyzing teacher learning.