This paper provides a methodologically original construction of Kant’s “Formula of Universal Law” (FUL). A formal structure consisting of possible worlds and games—a “game frame”—is used to implement Kant’s concept of a maxim and to define the two tests FUL comprises: the “contradiction in conception” and “contradiction in the will” tests. The paper makes two contributions. Firstly, the model provides a formal account of the variables that are built into FUL: agents, maxims, intentions, actions, and outcomes. This establishes a clear benchmark for understanding how the mechanics of FUL actually work. Secondly, the analysis of the resulting framework sheds new light on discussions about the implications of FUL. On the basis of this, we suggest a move to “comprehensive Kantianism’, which is the application of FUL to systems of maxims rather than to isolated maxims.