An Argument against Moral Nihilism

The following is an adaption of a friend’s arguments against error-theory.  Error theory is the position that all moral statements are false.  It isn’t the only variant of moral nihilism, but I think it’s the most competent.  If we can give a strong reason to doubt that it’s coherent, that’ll give us a strong reason to adopt moral realism.  

Error-theory: All predications of objective moral value are false.

We can show that this position results in a contradiction.  

1. Assume error theory is true. 
2. Which means that the following is true: If all moral predications are false, and P is some moral predication, then P is false. 
3. Yet if we think that at least some negations of moral predications are themselves moral predications, then we also have that the negation of P is false.   
4.  Error theorist must maintain that all negations of moral predications cannot themselves be moral predications.
5.  But we have a counter-example that shows that a negation of a moral predication is itself a moral predication.  
6.  “P and not P” both being simultaneously true is a contradiction, and any theory that implies a contradiction is false.
7.  Error-theory implies a contradiction, and is thus false.  

But it’s worse than this.  If the error theorist wants to maintain that there is a translation of ¬P that is not a moral predication, he is committed to the position that no proposition of the form ¬P is a moral predication, because otherwise his theory still implies a contradiction. He must then say that propositions of the form “it is not the case that X is morally wrong” are necessarily not moral predications.  

So the error theorist is committed to an even stronger thesis than with which he started: 

Revised Error-Theory (RET): All statements of predication of objective moral value are necessarily false.

RET is the only way to block the negation entailments of moral predications, that is, that “M is not wrong” entails “M is right” are all false.  On its own, this is too strong.   If we even have one counter-example, the theory is defeated.  As shown below, there is a counterexample.  

In support of premise 5, take the following example:

M: It is not the case that it is not permissible to murder.

This seems, on the face of it, to be a moral predication. It asserts that it’s not true that it is impermissible to murder. 
Since M is a moral statement, the error-theorist believes that M is false.  So the error theorist accepts ~M.  Let’s go through the steps here:
~M: It is not the case that it is not the case that it is not permissible to murder
Which means:  It is the case that it is not permissible to murder.
Which obviously is a moral predication.  So we have a counterexample. 

And by modus tollens;

8. If Revised Error-Theory is true, then there are no negations of moral predications that are themselves moral predications.
9.  But ~M is a moral predication that is itself a negation of a moral predication.
10. So Revised Error-Theory is false.