A ʻsystemʼ, as the concept was devolped by Firth, can be interpreted as the set of options that is specified for a given environment. The meaning of it is ʻunder the conditions stated, there are the following possibilitiesʼ. By making use of this notion, we can describe language in the form of a behaviour potential. In this way the analysis of language comes within the range of a social theory, provided the underlying concepts of such a theory are such that they can be shown to be realized in social context and patterns of behaviour.
- Halliday (1973), a pag.51

A system is an abstract representation of a paradigm; and this, as we have noted, can be interpreted as a set of options with an entry condition–--a number of possibilities out of which a choice has to be made if the stated conditions of entry to the choice are satisfied. It has the form: if 'a', then either 'x' or 'y' (or. . .). The key to its importance in the present context is Firth’s ʻpolysystemic principleʼ, whereby (again following this interpretation) the conditions of entry are required to be stated for each set of possibilities. That is to say, for every choice it is to be specified where, under what conditions, that choice is made. The ʻwhereʼ, in Firth’s use of the concept of a system, was ʻat what point in the structureʼ; but we interpret it here as ʻwhere in the total network of optionsʼ. Each choice takes place in the environment of other choices. This is what makes it possible to vary the ʻdelicacyʼ of the description: we can stop wherever the choices are no longer significant for what we are interested in.
- Halliday (1973), a pag.55