Another difference is that Parsons asks how certain subsystems contribute to the functioning of overall society. Luhmann starts with the differentiation of the systems themselves out of a nondescript environment. While he does observe how certain systems fulfill functions that contribute to "society" as a whole, he dispenses with the assumption of "a priori" cultural or normative consensus or "complimentary purpose" which was common to Durkheim and Parsons' conceptualization of a social function. For Luhmann, functional differentiation is a consequence of selective pressure under temporalized complexity, and it occurs as function systems independently establish their own ecological niches by performing a function. Functions are therefore not the coordinated components of the organic social whole, but rather contingent and selective responses to reference problems which obey no higher principle of order and could have been responded to in other ways.