Against Selective Realism (given methodological naturalism)
The most popular versions of realism in the scientific realism debates today are species of selective realism. A selective realist does not hold that mature, widely accepted scientific theories are, taken as wholes, approximately true---rather, she holds that (at least for some theories) only certain parts are approximately true, but others parts are not, and thus do not merit rational belief. The key question selective realists have grappled with over the last few decades is: which are the 'good' parts (the "working posits," in Kitcher's widely used terminology) and which are the 'bad' parts (the "idle wheels") of a theory?
An argument against any sort of philosophical selective realism just occurred to me, and I wanted to try to spell it out here. Suppose (as the selective realist must) there is some scientific theory that scientists believe/ accept, and which according to the selective realist makes at least one claim (call it p) that is an idle wheel, and thus should not be rationally accepted.
It seems to me that in such a situation, the selective realist has abandoned (Quinean) methodological naturalism in philosophy, which many philosophers---and many philosophers of science, in particular---take as a basic guideline for inquiry. Methodological naturalism (as I'm thinking of it here) is the view that philosophy does not have any special, supra-scientific evidential standards; the standards philosophers use to evaluate claims should not be any more stringent or rigorous than standards scientists themselves use. And in our imagined case, the scientists think there is sufficient evidence for p, whereas the selective realist does not.
To spell out more fully the inconsistency of selective realism and methodological naturalism in philosophy, consider the following dilemma:
By scientific standards, one either should or should not accept p.I can imagine at least one possible line of reply for the selective realist: embrace the parenthetical remark in the first horn of the dilemma above, namely, scientists are making a mistake by their own lights in believing p. Then the selective realist would need to show that there is a standard operative in the scientific community that the scientists who accept p don't realize should apply in the particular case of p. But that may prove difficult to show at this level of abstraction.
If, by scientific standards, one should not accept p, then presumably the scientific community already does not accept it (unless the community members have made a mistake, and are not living up to their own evidential standards). The community could have re-written the original pre-theory accordingly to eliminate the idle wheel, or they could have explicitly flagged the supposed idle wheel as a false idealization, e.g. letting population size go to infinity. But however the community does it, selective realism would not recommend anything different from what the scientific community itself says, so selective realism becomes otiose ... i.e., an idle wheel. (Sorry, I couldn't help myself.)
On the other hand, if, by scientific standards, one should accept p, then the selective realist can't be a methodological naturalist: the selective realist has to tell the scientific community that they are wrong to accept p.