 In order to sort out how supervenience connects up to other relations—like entailment, reduction, grounding, ontological dependence, and explanation—we need to discuss the fact that supervenience can hold with varying degrees of modal force. That is, the ‘cannot’ in “there cannot be an A-difference without a B-difference” comes in different strengths. For example, it can mean “cannot as a matter of logic,” or it can mean “cannot consistently with the laws of nature”.