II. The argument from nonphysical realitiesReppert's argument is similar to Mackie's argument from the "queerness" of objective moral values. Mackie's version seems to me to amount to little more than an argument from personal incredulity: "Well, I don't see any way such bizarre properties could exist!"
1. Probably, there are no realities that are not physical in nature; that is, that do not exist at particular places and times and are not complex states of fundamental physical particles.
2. If objective moral values exist, then there would be realities that are not physical in nature.
3. Therefore, probably, there are no objective moral values.
Reppert's version has the virtue of being much more specific. However, it runs us into all the difficulties of providing a physicalist description of phenomena. Moral values are no longer quite so alone, quite so "queer." What about other non-physical realities, like thoughts, emotions, and sensations? All of these would fall to the same objection - or, if a physical realization can be provided for these entities, why couldn't one also be provided for moral values?
Or take the realm of logic and mathematics. We know that if A implies B and B implies C, then A implies C.We can't justify this intuition in a rigorous way - we can only take it as an axiom. (As the tortoise pointed out to Achilles.) However, mathematical logic seems to be completely objective: everyone who takes the trouble to study and understand the subject pretty much agrees on the results. It seems possible, at least, that morality is something like logic. Perhaps there is some set of self-evident axioms that everyone can agree on, from which all of moral thought follows logically.
The subjectivist can argue that, in fact, there is no such set of agreed-upon axioms, unlike in logic. But that's a different objection.