Andrew Huddlestone offers a good treatment of Parfit on Nietzsche.
- Parfit really wants convergence because he is committed to moral intuitionism in the tradition of Sidgwick. On this picture, epistemic peers in ideal conditions should converge on a cluster of “self-evident” normative axioms—on pain of intractable disagreement about whose “self-evident” intuitions to trust.
- Compare mathematics, geometry and logic, where there does seem to be a common core of widely shared “self-evident” intuitions, that can be taken as axioms (even though non-Western thought has some variations).
- Parfit takes Nietzsche seriously, thinks of him as an epistemic peer on par with (e.g. Kant). So he needs to either dissolve merely apparent disagreement, or to explain Nietzsche’s disagreement in terms of non-ideal epistemic conditions or mistakes that Nietzsche would have recognised as such, if they were pointed out.
- Parfit tries to paint Nietzsche as a mixture of (a) more in agreement than he seems and (b) making a couple of basic mistakes. To do this, he strawmans Nietzsche rather badly, making heavy use of unpublished journal fragments (an approach which strikes me as odd… to the point of underhand?).
- Parfit doesn’t give a plausible account of Nietzsche’s normative views (anti-egalitarianism; suffering sometimes non-instrumentally good) or his meta-axiological views (Huddlestone thinks they’re underdetermined by Nietzsche’s writings, but Nietzsche definitely didn’t hold the “normativity requires God” thesis which Parfit attributes to him).