Social systems are self-referential systems based on meaningful communication. They use communication to constitute and interconnect the events (actions) which build up the systems. In this sense they are "autopoietic" systems. They exist only by reproducing the events which serve as components of the system. They consist therefore as events, i.e. actions, which they themselves reproduce and they exist only as long as this is possible. This, of course, presupposes a highly complex environment. The environment of social systems includes other social systems, (the environment of a family includes for example other families, the political system, the economic system, the medical system, and so on). Therefore communications between social systems is possible; and this means that social systems have to be observing systems, being able to use, for internal and external communication, a distinction between themselves and their environment, perceiving other systems within their environment.
Luhmann, Niklas "The World Society as a Social System," in International Journal of General Systems Vol. 8, No. 1 1982:131