This article deals with some long-standing philosophical issues concerning the nature of educational theory, its relevance to educational practice and its role in the professional development of teachers. Instead of treating these questions as "theoretical", however, it approaches them through a reflective autobiographical account of the role that "theory" has played in my own professional development as an educational theorist. Focusing in this way on the practice of theorising not only reveals a conception of the theory-practice relationship that is very different from that implicitly presupposed in the practice of most educational theorists (including educational philosophers); it also suggests that the most appropriate way to understand the role of educational theory in the professional development of teachers is by reference to the Aristotelian notion of practical reasoning and the now discarded tradition of "practical philosophy".