A distinction splits the world in two parts, "That" and "This, or "environment" or "system", or "us" and "Them" (IESC, EA)

It is the very fundamental act of system theory, the very act of

Distinctions coexists with purposes. (IESC)

Certainly, it is the most fundamental act of system theory, the very act of defining the system presently of interst, of distinguishing it from its environment (EA)

A distinction can be seen as an element of cognitive structuration. Indeed, any process of perception implies a classification between phenomena. This classification operation has two aspects :

1.the phenomena which are put together in a class, are considered to be equivalent with respect to the observer's goals, they are assimilated, they belong to the same equivalence class ;

2.the phenomena corresponding to different classes are distinguished or discriminated, they belong to different equivalence classes.

The operations of distinction, and assimilation of phenomena necessarily go together. If a cognitive system would make no distinctions, only assimilations, it would be unable to perceive different phenomena, it would react to all situations in a uniform way ; hence, it would be unable to adapt to a changing environment. On the other hand, a system which would make no assimilations, only distinctions, would be unable to anticipate; hence it would also be unable to adapt.

Spencer-Brown (1969) has proposed general axioms for distinctions. With these axioms, he has shown that a set of distinctions has a Boolean algebra structure, isomorphic to the algebra of classes in set theory or to the algebra of propositions in logic (Spencer-Brown, 1969). Spencer Brown showed that distinction algebra implies propositional calculus. B. Banaschewski (1977) showed the opposite entailment in *B. Banaschewski (1977), Notre Dame Journal of Formal Logic 3: 507-509.

Distinction constrains the domain of discourse, because the act of distinguishing specifies (even if only implicitly) both something referred to and the context in which it is manifest. Because a unity is brought forth only through distinction, "...each time we refer to a unity in our descriptions, we are implying the operation of distinction that defines it and makes it possible." (Maturana & Varela, 1992, p. 40)

2. The act or process by which a unity effects or asserts its own separability from its ambience or background. This sense of "distinction" highlights the manner in which an autonomous / autopoietic system establishes, as the topological manifestation of its organizational closure, a literal or functional boundary delineating its extent with respect to its ambience. Varela differentiates this sense of the term from 1. above by labeling it physical (e.g., when realized in the physical space). (1979, pp. 30-31)

This concept has been discussed by J.A. GOGUEN and F. VARELA who state: "A distinction splits the world into two parts, "That" and "This", or "environment" or "system", or "us" and "them", etc. One of the most fundamental of all human activities is the making of distinctions. Certainly, it is the most fundamental act of systems theory, the very act of defining the system presently of interest, of distinguishing it from its environment. Distinctions coexist with purposes". (1967, p.32) The corresponding logical model is based on SPENCER BROWN’s distinction algebra. (1969)

It is also useful in order to differentiate causal and stochastic processes, as well as reversible (retrodictable) or irriversible (and more or less predictable) ones.