We have suggested elsewhere (Gaines, 1980) that
what is distinguished is a system and that
this notion is a sufficient definition for a system. A system is what is distinguished as a system and
in this lies the essence of systems theory. To distinguish some entity is a necessary and sufficient criterion for its being a system and this is uniquely true for systems. This definition may appear too weak in that it does not involve the expected properties of a system. The Oxford English Dictionary notes that a system is "a group, set or aggregate of things, natural or artificial, forming a connected or complex whole." However, these are properties that derive from our definition: a set of things is treated as distinct from another and it is this which gives it coherence; it is also this which increases its complexity by giving it one more characteristic than it had before, that it has now been distinguished.
Thus,
the notion of a distinction leads naturally to that of a system and the axiom of comprehension is equivalent to the assumption that all distinctions define systems.
http://pages.cpsc.ucalgary.ca/~gaines/reports/SYS/SGSR84B/index.html
Gains 1980 Gaines, B.R. (1980). General systems research: quo vadis? Gaines, B.R., Ed.
General Systems 1979. pp.1-9. Kentucky, Society for General Systems Research
