In the first place language serves for the expression of content: it has a representational, or, as I would prefer to call it, an IDEATIONAL function [...] Two points need to be emphasized concerning this ideational function of language.The first is that it is through this function that the speaker or writer embodies in language his experience of the phenomena of the real world; and this includes his experience of the internal world of his own consciousness: his reactions, cognitions and perceptions, and also his linguistic acts of speaking and understanding [...] There is, however, and this is the second point, one component of ideational meaning which, while not unrelatable to experience, is nevertheless organized in language in a way which marks it off as distinct: this is the expression of certain fundamental logical relations such as are encoded in language in the form of co-ordination, apposition, modification and the like [...] Within language, therefore, we can recognize two sub-functions, the EXPERENTIAL and the LOGICAL and the distinction is a significant one for our present purpose.
- Halliday (1973), a pag.105

Here we are referring to the linguistic expression of ideational content; let us call this macro-function of the adult language system the ʻideationalʼ function. For the child, the use of language to inform is just one instance of language use, one function among many. But with the adult, the ideational element in language is present in all its uses; no matter what he is doing with language he will find himself exploiting its ideational resources, its potential for expressing a content in terms of the speaker’s experience and that of the speech community.
- Halliday (1973), a pag.37

The ideational function [...] is a major component of meaning in the language system that is basic to more or less all uses of language. It is still a ʻmeaning potentialʼ, although the potential is very vast and complex; for example, the whole of the transitivity system in language---the interpretation and expression in language of the different types of process of the external world, including material, mental and abstract processes of very kind---is part of the ideational component of the grammar. And the structures that express these ideational meanings are still recognizably derived from the meanings themselves; their elements are in this respect not essentially different from those such as ʻobject of desireʼ [...] They represent the categories of our interpretation of experience [...] Hence this function of language, which is that of encoding our experience in the form of an ideational content, not only specifies the available options in meaning but also determines the nature of their structural realizations.
- Halliday (1973), a pag.38-39