 How plausible is Mackie’s conceptual claim? This issue cannot be discussed in detail here, except to note that while it seems plausible to claim that if our concept of a moral fact is a concept of a reason for action then that concept must be a concept of a categorical reason for action, it is not so clear why we have to say that our concept of a moral fact is a concept of a reason for action at all. If we deny this, we can concede the conditional claim whilst resisting Mackie’s conceptual claim. One way to do this would be to question the assumption, implicit in the exposition of Mackie’s argument for the conceptual claim above, that an ‘ought’-statement that binds an agent A provides that agent with a reason for action. For an example of a version of moral realism that attempts to block Mackie’s conceptual claim in this way, see Railton (1986). For defence of Mackie’s conceptual claim, see Smith (1994), Ch.3 and Joyce (2001). For exposition and critical discussion, see Miller (2013a), Ch.9.