This paper is about whether we can revise the highest rule in a legal system using legal reasoning. HLA Hart and Alf Ross argued that we can’t: either the highest rule is unchangeable, or it can only be changed in a revolution—a merely causal change, not a legal one. I argue, drawing on an idea from Hartry Field about logic and epistemology, that we can. The emergence of the conclusion, within a legal system, that the highest rule should be replaced by something else can provide a legal reason for the change. In making the change, we act for reasons internal to the legal system. This allows us to make sense of the distinction, recognized by legal subjects, between a revolution and a legal change of sovereignty.