We ordinarily distinguish between the authority exercised by an expert and that exercised by a commander. Nevertheless, prior argumentation theorists have been unable to articulate fully the grounds on which we make this distinction. In this paper, I propose a principle for distinguishing types of authorities. I argue further that on this principle, Locke's argumentum ad verecundiam represents a third type, reducible neither to command nor expertise. Finally, I point to significant instances of this third appeal to authority, especially in Roman legal and political discourse.