American philosopher Wilfrid Sellars (1962) once said that “the aim of philosophy, abstractly formulated, is to understand how things in the broadest possible sense hang together in the broadest possible sense.” My main question is this: within the context of contemporary sustainability science, how does the concept of ‘sustainability’ in the broadest possible sense of the concept hang together in the broadest possible sense? I will answer this question by advancing two new explicative definitions of sustainability that jointly constitute a (...) unifying concept of sustainability. This meta-level concept accommodates most of the central meanings conventionally assigned to ‘sustainability’ by sustainability scientists and scholars, offers a useful division of labor between scientists and scholars, and makes explicit value judgements commonly associated with the concept of sustainability. (shrink)