Niklas Luhmann was a prominent sociologist and social systems theorist who laid the foundations of modern social system thought.[5] He based his definition of a "social system" on the mass network of communication between people and defined society itself as an "autopoietic" system, meaning a self-referential and self-reliant system that is distinct from its environment.[6] Luhmann considered social systems as belonging to three categories: societal systems, organizations, and interaction systems.[7] Luhmann considered societal systems, such as religion, law, art, education, science, etc., to be closed systems consisting of different fields of interaction.[8] Organizations were defined as a network of decisions which reproduce themselves; his definition is difficult to apply in terms of finding a real-world example.[8] Finally, interaction systems are systems that reproduce themselves on the basis of communication rather than decision making.[8]