Derek Parfit has recently circulated an argument against what he calls Non-Analytical Naturalism, which he understands as the thesis that normative truths are reducible to natural truths. He begins by stipulating that he will use 'normative' as an abbreviation for 'irreducibly normative':
When some normative word cannot be analyzed or defined in non-normative terms, we can call this word, and the concept it expresses, irreducibly normative. That is what I shall mean by 'normative'...
His central argument then appears to be that Non-Analytic Naturalism is (by definition) inconsistent with normativity, and hence false.
Now, I have great sympathy for the idea that it should be a test of adequacy on any reductive theory, that it can account for the normativity of wrong, good, and so on. And I think it is a quite interesting question whether any reductive theory can account for this, and moreover what constraints, if any, would be placed on a reductive theory that would be able to do so. I simply don't see, however, how Parfit's argument contributes anything at all to this investigation, or in fact amounts to anything other than a stipulation that he is going to use words in a way that presupposes his conclusion.
It's easy to stipulate that by 'normative' we will mean, inter alia, 'something that a reductive theory cannot account for'. That makes the step to the conclusion that reductive theories cannot account for normativity trivial. The interesting question is whether the senses of normative words that Parfit claims are irreducible are in fact so.
If anyone sees anything that look more like an argument lying behind Parfit's remarks, I'm very much hoping to discover what it is supposed to be.