The Argument from First-Order Ethical Beliefs for Moral Realism

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Definitions

first-level moral belief: a belief that something is good or evil or that something is of equal or greater value than something else
second-level moral belief: a belief about some or all first-level moral beliefs

The Argument 

As far as I can tell, the argument from ordinary language originated with John Post, but has also been defended by Quentin Smith.  The argument begins with the observation that “our first-order ethical beliefs imply that ethical sentences have truth-value and sometimes correspond to moral facts that obtain independently of our beliefs about whether they obtain” (p. 158).  For example, when the average person says, “Rape is morally wrong,” they typically do not mean that rape is wrong for them but okay for rapists.   On the contrary, they mean that rape is morally wrong for everyone.  Or, similarly, it violates ordinary usage to say, “It all comes down to a personal decision in the end as to whether it is wrong to torture newborn babies for the fun of it.”  More formally, we may say:

(1)   Ordinary ethical sentences and commonsense first-level moral beliefs imply moral realism (or “Moral realism tacitly seems to be true in ordinary commonsense moral attitudes”).

Smith then appeals to an epistemic thesis called the principle of veridical seeming.  According to this principle, we have reason to believe what seems to us to be true, unless there is good reason for us to distrust what seems to us to be the case.  With respect to moral realism, there are no empirical reasons to believe moral realism is false.  (Elsewhere, Smith provided refutations of Mackie’s arguments from queerness and disagreement.)  There are no a priori analytic reasons to believe moral realism is false, since propositions asserting the existence of moral facts are not implicitly logically contradictory.  There are no a priori synthetic reasons to believe moral realism is false.  (At least, I’m not aware of any philosopher who has ever offered such a principle.)  This provides us with the following premise:

(2)   There are no empirical or a priori reasons to believe that first-level moral beliefs are all false.

 Thus, on the basis of premises (1) and (2), we may conclude:

(3)   It is more reasonable to believe moral realism than not to believe this.

One might attempt to deny that (1) and (2) entail (3) on the basis of some principle that contradicts the principle of veridical seeming.  For example:

(S) If p seems to be true, then, even in the absence of empirical and a priori reasons to not believe p, p’s seeming to be true does not justify a belief in p.

As Smith asks, what would justify a belief in the skeptical principle (S)?  If there are no a priori or empirical reasons to not believe (S), then the skeptic cannot appeal to the fact that (S) appears true to her to justify her belief in (S).  As Smith puts it, “For (S) rules out the epistemic legitimacy of this very appeal.”  Of course, the skeptic could try to introduce some secondary reason, (R), as a reason to believe (S).  If so, then the skeptic could argue that it appears to the skeptic that (R) is true and that (R) justifies (S).  But an affirmation of (S) rules out such a move: according to (S), what appears to the skeptic to be the case provides no reason to believe that is the case.  Thus, in order to be consistent, the skeptic cannot trust her own beliefs, including her beliefs about the justification of (S), and thus her belief in (S) is self-defeating. 
Moreover, the argument-form (1)-(3) is self-justifying: it seems to be valid, there are no a priori or empirical reasons to believe it is invalid, and therefore it is justified to believe it is valid.  More formally:
(4)   There is no reason to believe that the conjunction of (1) and (2) is a defective reason to believe moral realism.
(5)   Therefore, the belief in moral realism is indefeasibly justified.
References

Post, John F. The Faces of Existence: An Essay in Nonreductive Metaphysics. Ithaca: Cornell University Press, 1987.

Smith, Quentin. Ethical and Religious Thought in Analytic Philosophy of Language (New Haven: Yale University Press, 1998),