Here are some premises:
- If x did A because s, and either (a) it is false that s or (b) the fact that s is not a part of any explanation of x's doing A, then x's doing A because s is defective.
- If naturalism is true, then a human's doing something is a natural state of affairs.
- If naturalism is true, then moral facts do not explain any natural state of affairs.
- That something is morally required is a moral fact.
- Some human being non-defectively did something because it was morally required.
- Naturalism is not true.
Premises 2 and 4 are hard to dispute. Premise 5 seems plausible: it would be very odd indeed if every case of acting from duty were defective, assuming of course morality is an objective fact. Premise 3 is a bit tougher. If moral facts reduce to natural facts, then there is no reason to assert 3. For instance, one might reduce "A is required" to "A maximizes utility" and then reduce utility to natural facts about desire or pleasure. Neither step in the reduction seems plausible to me, though both have been defended. Now, naturalism doesn't want there to be non-natural explanations of natural facts. Natural facts either have no explanation or have only a natural explanation, according to the naturalist. So, unless there is a reduction of moral facts to natural ones, 3 is pretty plausible.
That leaves premise 1. I may have a counterexample to premise 1. Suppose I know that it will rain tomorrow, so today I buy an umbrella. It seems that I bought the umbrella because it will rain tomorrow, but the fact that it will rain tomorrow is not a part of any explanation of my buying the umbrella. If this case is non-defective, then 1 is false. However, perhaps, there is something rationally defective in this case. For, perhaps, my reason for buying the umbrella shouldn't be tomorrow's rain, but that the forecast predicts rain.
Currently, I am inclined against 1.