  Eschewing Lewis’s game-theoretic orientation, Margaret Gilbert (1989) instead draws inspiration from sociology, specifically from Georg Simmel’s theory of “social groups” (1908). The basic idea is that individual agents can “join forces” to achieve some common end, thereby uniting themselves into a collective entity. To develop this idea, Gilbert provides a complex account of how agents bind themselves into a “plural subject” of belief and action. A plural subject is a set of agents who regard themselves as jointly committed to promoting some goal, sharing some belief, or operating under some principle of action. By virtue of their common knowledge of this joint commitment, members of the plural subject regard themselves as “we”. They thereby regard one another as responsible for promoting the group’s goals and principles. For instance, two traveling companions make manifest their commitment to keep track of one another, whereas two people who happen to share a seat on a train do not. The traveling companions form a plural subject. The unaffiliated travelers do not. The traveling companions regard each other as responsible for helping if one person falls behind, for not losing one another in a crowd, and so on.