- Moral language requires categorical imperatives.
- Categorical imperatives cannot be legitimately questioned.
- Practical rationality is the only source of statements that cannot be legitimately questioned.
- But practical rationality cannot provide a basis for (moral) categorical imperatives.
- Therefore, moral language is in error.
One way to avoid (4) is to object to Joyce's view of practical rationality. Joyce spends a whole chapter (Ch. 5) answering this objection. His approach is to "attempt a straight defense of practical instrumentalism [his version of practical rationality] by showing that the non-instrumentalist necessarily commits an error." The argument he uses is based on a "well-known" paper by Bernard Williams, "Internal and External Reasons."
Williams's conclusion is that something is a reason if (and only if), after a process of fully informed and correct deliberation, it would motivate someone to act in accordance with the reason. Williams thus denies that there are such things as "external" reasons. (In the lingo, "internal" reasons are those that are motivating, "external" reasons are those that are not.)
Joyce is more circumspect. He doesn't agree that Williams's conclusion applies to all reasons. But he does think it applies to the sort of normative reasons that are needed for moral language.
My objection is only with external reason claims that do not know their place - that overstep themselves by claiming to transcend all institutions.
Once again we see the importance to Joyce of the imperative that is categorical, that "transcends all institutions."
If we go along with Joyce's definitions and assumptions, then his error-theory conclusion is unavoidable. Joyce admits that he has not "proven" error theory - he has only tried to make it probable. His next move, though, is pretty weird: he tries to treat morality as a work of fiction. More on that next time!