Researchers can deepen investigations of college teaching and learning by attention on how students construe a lesson's subject matter and on how teachers make sense of and respond to their students' subject-matter thinking in the moment. While it is possible to examine teaching and learning minus subject-matter depth, doing so obscures the dynamics that define education. My illustrative example captures teaching and learning about the idea of Cartesian doubt in an introductory philosophy class. I analyze students' struggles with this new (and to most, startling) idea in light of their prior knowledge. I also examine the instructor's efforts to grasp and respond to students' evolving conceptions of Cartesian doubt. (Contains 2 footnotes.)