Systems of mutually defining entities

Saussure, in conceiving what language is, said that (as summarised by David Crystal) it is a system of mutually defining entities.

Language is but one such system.

When the entirety of the reality is taken to be closed, it is system as well. But it is a completely different story whether we label every single object in the reality.

I like the word entity (something that exists as a single and complete unit). What counts as a single unit is a difficult question. Letters maybe taken as a single unit but they are without meaning. The inventory of letters in English is a system. The other inventory of its words is another system. The inventory of sentence patterns is a system, and so on. The point is there are systems within systems, and systems are defined by the observer as he or she needs the system. But whether the inventory of entities are valid or not it is up to the definition and acceptance of those people or things involved.

My position is that certain systems include inventory that I would not consider to be there, and that therein lies the problem. To define what is part of the inventory of a system is the first task.