A system may actually exist as a natural aggregation of component parts found in Nature, or it may be a man-contrived aggregation – a way of looking at a problem which results from a deliberate decision to assume that a set of elements are related and constitute such a thing called 'a system.

Churchman, C. W., I. Auerbach, and Simcha Sadam (1975) Thinking for Decisions Deduction Quantitative Methods. Science Research Associates. cited in: John P. van Gigch (1978) Applied General Systems Theory. Harper & Row Publishers