What is reasoning? Piaget and his collaborators have described a developmental and epistemological approach (Inhelder & Piaget, 1958). Information processing theorists describe mental activity in terms of conditions and “actions” called forth without further intermediate steps by these conditions, where “actions” may be overt behaviours or mental operations (Newell, 1973). To the classroom teacher, both of these points of view probably appear remote and theoretical, far from the comments, questions, insights, and misunderstandings exhibited by their students. This article, an abbreviated version of a longer presentation (Karplus, 1980), attempts to present an intermediate level of analysis, close enough to classroom occurences to be easily applied, but also sufficiently general to permit the linking of student behaviour with teaching strategies.