In order to delimit the realm of social phenomena, sociologists refer implicitly or explicitly to a distinction between living human beings and other entities, that is, sociologists equate the social world with the world of living humans. This consensus has been questioned by only a few authors, such as Luckmann, and some scholars of science studies. According to these approaches, it would be ethnocentric to treat as self-evident the premise that only living human beings can be social actors. The methodological (...) consequence of such critique is a radical deanthropologization of sociological research. It must be considered an open question whether or not only living human can be social actors. The paper starts with a discussion of the methodological problems posed by such an analysis of the borders of the social world, and presents the results of an empirical analysis of these borders in the fields of intensive care and neurological rehabilitation. Within these fields it must be determined whether a body is a living human body or a symbol using human body. The analysis of these elementary border phenomena challenges basic sociological concepts. The relevant contemporary sociological theories refer to a dyadic constellation as the systematic starting point of their concept of sociality. The complex relationship between at least two entities is understood as the basis of the development of a novel order that functions as a mediating structure between the involved parties. Based upon empirical data, I argue that it is necessary to change this foundational assumption. Not the dyad but the triad must be understood as the foundational constellation. This implies a new understanding of the third actor, which is distinct from the concepts developed by Simmel and Berger and Luckmann. (shrink)