Talcott Parsons was the first to formulate a systematic theory of social systems, which he did as a part of his AGIL paradigm. He defined a social system as only a segment (or a "subsystem") of what he called action theory.[4] Parsons organized social systems in terms of action units, where one action executed by an individual is one unit. He defines a social system as a network of interactions between actors.[4] According to Parsons, social systems rely on a system of language, and culture must exist in a society in order for it to qualify as a social system.[4] Parsons' work laid the foundations for the rest of the study of social systems theory and ignited the debate over what framework social systems should be built around, such as actions, communication, or other relationships.