A system is nothing more and nothing less than a set of relatively stable interactions.
A system can be evaluated only from the outside.
A system exists only in the mind of the observer.

I’ll discuss these three affirmations from the bottom.

Even if the objects that compose the system exist ‘in the real world’, the system itself exists only in the minds of the observers – the person who had first identified the interactions and all those who agree with him.
To illustrate my point I’ll give you three examples.
–  A constellation is “a group of stars that forms a particular shape in the sky and has been given a name“. A long time ago, when those constellations had been named by our ancestors, people thought they were something totally different than what we think of them now. Yet they continue to exist as before and…

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