Nothing is perfect. The #system (and consequently the #model built upon it) is always a selected subset of all the #variables available for the #control of the #machine.
#WR_Ashby in "Design for a Brain" (chapter 2/5) explains in more detail the meaning of the word "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’. Throughout the book, ‘the system’ will always refer to this abstraction, not to the real material ‘machine’.
As you can never control **all** the variables of any given "machine", there is always the possibility that the variables that you don't control will generate some unforeseen consequences.
