No matter how abstractly formulated are a general theory of systems, a general theory of evolution and a general theory of communication, all three theoretical components are necessary for the specifically sociological theory of society. They are mutually interdependent.

Luhmann (1982) The Differentiation of Society, Translated by Stephen Holmes and Charles Larmore. Columbia University Press, New York, 1982, pp. 261. Cited in: Loet Leydesdorff (2000) "Luhmann, Habermas, and the Theory of Communication"