This paper demonstrates how questions of "provability" can help students engaged in reinvention of mathematical theory to understand the axiomatic game. While proof demonstrates how conclusions follow from assumptions, "provability" characterizes the dual relation that assumptions are "justified" when they afford proof of appropriate results. I provide examples of students' learning in teaching experiments connected to an advanced, undergraduate, neutral axiomatic geometry course. Students used "provability" relations to choose definitions, develop the notion of independence, and understand the relationship between axioms and theorems. I argue that provability promotes cognitive need for systematization, allaying concerns that geometric proof lacks illuminative value.