This article presents a form of teacher reasoning that we call "proclivities for teaching mathematics." We define proclivities for teaching mathematics as the beliefs, knowledge, and dispositions that are actionable in the flow of instruction, and we argue that growth in this area contributes to positive change in mathematics instruction. Proclivities for teaching mathematics encompass four domains: productive disposition, openness to new ideas, thought processes, and organization of mathematical thought; in this article, we focus on productive disposition. The article provides a rationale for the construct of proclivities for teaching mathematics, and describes an initial attempt to develop measures for these proclivities. Analyses of data from a sample of 63 teachers indicate that measures of proclivities for teaching mathematics are associated with increases in teacher knowledge, self-efficacy, and instructional improvement over time, and although the dataset is small, these positive indicators suggest that further exploration of this construct is warranted.