Both popular and scientific definitions of life must account for the possibility of the sub-optimal operation of some function. Identifying the function in question and the criteria for optimality will be necessary steps in crafting a definition that is both intuitive and rigorous. I lay out a rule of thumb—the proper activity criterion—and a three-part typology of binary, range, and preference for understanding definitions of life. The resolution of “optimal” function within a scientific framework presents the central challenge to creating a successful definition of life. A brief history of definitions of life and explanations of biological function is presented to demonstrate the value of the typology. After analyzing three controversial cases—viruses, mules, and stars—I present three possible options for resolution: vitalism, reductionism, and instrumentalism. Only by confronting the consequences of each can we come to consensus about what is necessary and desirable from a common definition.