Therefore one negated the conception of the closed system for the area of biology and sociology and developed instead a theory of open systems: open systems because one wanted to explain why entropy does not occur and why order is built up.
Now openness means in any case exchange with the environment, but depending on whether one thinks of biological or organic systems or of sense-oriented systems, i.e. of social systems (communication systems) and of mental systems (consciousness and the like), this idea of Exchange takes different forms. For biological systems, one thinks primarily of energy input and output of useless energy; for sense systems, one thinks primarily of exchange of information. A sense system draws information from its environment, interprets, if we may say so, surprises, and is built into a network of other systems that responds to this information-processing system. The basic condition that explains entropy is the same in both cases, namely exchange relations between system and environment. This is what the term open system refers to.
> I emphasize this at this point, because later we will talk about a counter-theory of the operational closed system, in which, however, this concept of openness is not revoked, but revised. In any case, open systems are the answer to the provocation which had started from the entropy law.
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Niklas Luhmann, Einführung in die Systemtheorie, hg. von Dirk Baecker, 2. Aufl, Sozialwissenschaften (Heidelberg: Carl-Auer-Verl, 2004), p. 45–46.
p. 59: Two questions have thus developed from the one question of what this open system is, namely, on the one hand, the question of how a difference between system and environment is generated and reproduced, and on the other hand, the question of which operation type does this and how it can be internally networked. How can an operation type internally recognize that certain operations belong to the system and other operations do not? This question has become significant, for example, for immunology, for the theory of the immune system, since the end of the 1960s.