 So our concept of a moral requirement is a concept of a categorically prescriptive requirement. But Mackie claims further that our concept of a moral requirement is a concept of an objective and categorically prescriptive requirement. What does it mean to say that a requirement is objective? Mackie says a lot of different-sounding things about this, and the following (as outlined in Miller 2013a) is by no means a comprehensive list (references are to Ch. 1 of Mackie 1977). To call a requirement objective is to say that it can be an object of knowledge (24, 31, 33), that it can be true or false (26, 33), that it can be perceived (31, 33), that it can be recognised (42), that it is prior to and independent of our preferences and choices (30, 43), that it is a source of authority external to our preferences and choices (32, 34, 43), that it is part of the fabric of the world (12), that it backs up and validates some of our preferences and choices (22), that it is capable of being simply true (30) or valid as a matter of general logic (30), that it is not constituted by our choosing or deciding to think in a certain way (30), that it is extra-mental (23), that it is something of which we can be aware (38), that it is something that can be introspected (39), that it is something that can figure as a premise in an explanatory hypothesis or inference (39), and so on. Mackie plainly does not take these to be individually necessary: facts about subatomic particles, for example, may qualify as objective in virtue of figuring in explanatory hypotheses even though they cannot be objects of perceptual acquaintance. But his intention is plain enough: these are the sorts of conditions whose satisfaction by a fact renders it objective as opposed to subjective. Mackie’s conceptual claim about morality is thus that our concept of a moral requirement is a concept of a fact which is objective in at least some of the senses just listed, while his ontological claim will be that the world does not contain any facts which are both candidates for being moral facts and yet which play even some of the roles distinctive of objective facts.