 In his argument for error theory, J. L. Mackie briefly claims that supervenience makes trouble for the realist. His quick argument can usefully serve as a prelude to the more detailed discussion to come. Mackie suggests that we think that actions have their ethical properties because they have some natural features. For example, we think a certain action wrong because it is cruel. He denies that this ‘because’ references a conceptual entailment, and thinks this raises two questions: (1) what sort of relation is the connection being referred to? And (2) how do we come to know that actions stand in this relation? (1977, 41). As it stands, Mackie’s questions serve more as a research agenda than an argument (for important recent discussion, see Olson 2014, §5.1). It appears plausible that realists should aim to have something illuminating to say both about the nature of the relation between the ethical and base properties, and a credible epistemology for how we come to know such relations. But Mackie’s questions do not yet constitute an argument that realists cannot achieve these aims.