  This argument can succeed only if supported by a plausible version of the ‘ought implies can’ principle. But in basing an agent’s capacity to act on her desires and dispositions, the version of the principle that the argument seems to presuppose treats ‘ought’, or the fairness of blame, as depending on the psychological capacity to act rather than on the mere physical capacity to act. Externalists would reject as implausible the psychological version of the principle, and therefore to assume it for purposes of an internalist argument would be question-begging against the externalist.