The core element of Luhmann's theory pivots around the problem of the contingency of meaning and thereby it becomes a theory of communication. Social systems are systems of communication, and society is the most encompassing social system. Being the social system that comprises all (and only) communication, today's society is a world society. A system is defined by a boundary between itself and its environment, dividing it from an infinitely complex, or (colloquially) chaotic, exterior. The interior of the system is thus a zone of reduced complexity: Communication within a system operates by selecting only a limited amount of all information available outside. This process is also called "reduction of complexity". The criterion according to which information is selected and processed is meaning (in German, "Sinn"). Both social systems and psychic systems (see below for an explanation of this distinction) operate by processing meaning.