"A measurable quantity which at every instant has a definite numerical value" (W.R. ASHBY, 1960, p.14).
ASHBY states: "… every real 'machine' embodies no less than an infinite number of variables, all but a few of which must of necessity be ignored… Faced with the infinite number of variables, the experimenter must, and of course does, select a definite number for examination – in other words, he defines an abstracted system…
"Because any real 'machine' has an infinity of variables, from which different observers (with different aims) may reasonably make an infinity of different selections, there must first be given an observer (or experimenter); a system is then defined as any set of variables that he selects from those available on the real 'machine'. It is thus a list, nominated by the observer, and is quite different in nature from the real 'machine'.(Ibid, p.15-16).
As to G. PASK, he put it in this way: "Attributes of an assembly are identified with the variables in a mathematical model to form a system" (1961 a, p.116).
Also G. KLIR states that "A variable is always an abstract (mathematical) thing. It may, or may not represent some observable or measurable attribute of the real world" (1991, p. 47).
As a consequence, systemic models, when formalized, are sets of interconnected variables.
As the concept of 'variable' implies the comparison of different values at different instants, it implies also by necessity a reflexion about the concept of time.
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Bertalanffy Center for the Study of Systems Science (2020). Title of the entry. In Charles François (Ed.), International Encyclopedia of Systems and Cybernetics (2). Retrieved from www.systemspedia.org/[full/url]
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