Parsons was heavily influenced by Durkheim and Max Weber, synthesizing much of their work into his action theory, which he based on the system-theoretical concept and the methodological principle of voluntary action. He held that "the social system is made up of the actions of individuals." His starting point, accordingly, is the interaction between two individuals faced with a variety of choices about how they might act, choices that are influenced and constrained by a number of physical and social factors.