  Gilbert proposes that “our everyday concept of a social convention is that of a jointly accepted principle of action, a group fiat with respect to how one is to act in certain situations” (p. 377). Members of a population jointly accept a fiat when it is common knowledge that they have made manifest their willingness to accept and promote that fiat as a basis for action. By Gilbert’s definition, participants in a convention constitute a plural subject. For they jointly accept the common goal of promoting some fiat. Moreover, members of the social group regard the fiat as exerting normative force simply by virtue of the fact that they jointly accept it. Note that not all plural subjects instantiate conventions. For instance, depending on the details of the case, the traveling companions from the previous paragraph might not. A convention arises only when individual members of a plural subject jointly accept some fiat.