 Over the last two decades, philosophers have done a great deal of work on the notion of grounding (see, e.g., Fine 2001, Rosen 2010, Audi 2012, Schaffer 2009 Koslicki 2012 and the Stanford Encyclopedia entry on  grounding).  It is thought by many currently working on issues in the metaphysics of grounding that grounding is a primitive, sui generis relation. In particular, it is not to be understood as a supervenience or causal relation. How are we then to understand what it is? Philosophers point to particular cases where it is instanced: Dispositional properties are grounded in categorical properties, the mental is grounded in the physical, the semantic is grounded in the non-semantic, features like smiles or surfaces are grounded in facts about bodies, and so on. To this point, one might think that grounding talk can be captured by our ordinary notion of  supervenience.[3]  But Fine (2001) claims that Socrates’ singleton set is grounded in Socrates; yet, necessarily one exists just if the other does. Thus our ordinary modal notion of supervenience won’t capture this case of grounding. If we assume (as many in the grounding literature do) that the other cases of grounding are of the same sort as the Socrates-singleton case is, then our ordinary notion of supervenience won’t capture them either. 