[T]he appeal to reason is implicitly authorized by the [subjectivist] challenge itself, so this is really a way of showing that the challenge is unintelligible. The charge of begging the question implies that there is an alternative—namely, to examine the reasons for and against the claim being challenged while suspending judgment about it. For the case of reasoning itself, however, no such alternative is available, since any considerations against the objective validity of a type of reasoning are inevitably attempts to offer reasons against it, and these must be rationally assessed. The use of reason in the response is not a gratuitous importation by the defender: It is demanded by the character of the objections offered by the challenger.

Thomas Nagel, The Last Word, New York, 1997, p. 24