Also, Timmons says, in connection with both of these arguments Mackie makes the point that a moral realist who countenances the existence of metaphysically queer properties, facts, and relations must also posit some special faculty by which we have knowledge of them (Timmons, p. 51).
In his 1977 book Ethics: Inventing Right and Wrong, Mackie uses these arguments to assert that most moral discourse is flawed because it presumes the existence of moral facts which are not true.