The concept of re-entry describes the ability of autopoietic systems [→Autopoiesis] – which are differentiated on the basis of a Distinction that allows the production of the unity of the system – to introduce this distinction into themselves and to use it to structure their operations.
It is a re-entry if, for instance, a functional system differentiated on the basis of the particular distinction of its →Code learns how to process this distinction internally: for instance, if the scientific system, whose operations are oriented to the code true/untrue, develops a scientific theory that observes the use of the code true/untrue using the code true/untrue.
Epistemological reflection is the re- entry of the distinction true/untrue into the system established on the basis of this distinction: thus, there is a scientific operation in which the truth of scientific operations—i.e., the truth of the distinction true/untrue—is questioned.
In this way, a situation arises in which the distinction is simultaneously the same (when it is the particular distinction of that system operations) and different (when it is the observed distinction). The problem that follows from this situation is how to handle this →Paradox without being blocked by it. The problem of re-entry is the “otherness of the same”: the necessity of processing the same distinction as if it were a different one.
Re-entry indicates the “re-introduction” of a distinction into a domain that is differentiated by the distinction itself. The term is derived from George Spencer Brown’s logical calculus [→Operation/Observation], a feature of which is that it is based exclusively on the operation of indication/distinction.
Systems theory interprets this operation as observation: something is indicated and at the same time distinguished from others things. The connections between operations within one and the same system lead to the construction of ever more complex forms, until the point at which the calculus has reached a sufficient level of complexity.
Then the system includes an operation that, in place of an external object, again indicates the system-constituting operation of indication/distinction,i.e., the same operation that the operation itself realizes.
Through recourse to time, the system is able to process this operation within itself. It is then possible to produce an (observational) operation that distinguishes its own distinction from something else—i.e., an operation in which the distinction appears twice, both as a system-specific distinction and as a running distinction; as observing distinction and observed distinction. Here we have a re-entry.
Usefulness of the concept of re-entry
The concept of re-entry is useful first and foremost in order to tackle the issue of the →Paradox, because it shows how a system can neutralize paradoxes through recourse to the temporal sequence of its operations.
It is also useful because it allows the possible binary distinctions [→Code] to be discriminated in terms of which of them are appropriate for guiding the autopoiesis of a system.
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Only distinctions capable of re-entry enable a minimal level of complexity to be overcome in the construction of a system. These distinctions are capable of processing the unity of the distinction on one of their sides. One such example is the distinction system/environment: once the system has reached a certain level of complexity, it is able to tackle the question of its own relationship to the environment.
The capacity for re-entry sets this distinction apart from alternatives such as, for instance, the distinction whole/parts. If we only had the distinction between the whole and its parts, it would not be possible to take the surplus into account, which makes the whole more than the mere sum of its parts. In order to qualify this surplus, we would need a term defined independently of the opposition of parts and whole: we would need recourse to another distinction. [E.E.] – (Unlocking Luhmann, p 195– 196)
Die Wissenschaft der Gesellschaft (1990: 83 ff., 479 ff.); The Paradox of Observ- ing Systems (1995); Observing Reentries (1993).