[...] the principle that a clause is the product of three simultaneous semantic processes. It is at one and the same time a representation of experience, an interactive exchange, and a message. At this point, however, we need to introduce more general functional concepts to which we can relate these three aspects of the meaning of the clause. These are the three kinds of meaning that are embodied in human language as a whole, forming the basis of the semantic organization of all natural languages. We shall refer to these as ʻmetafunctionsʼ, and use for them the terms IDEATIONAL, INTERPERSONAL and TEXTUAL.
- Halliday (1985), a pag.53

[...] The English clause is a composite affair, a combination of three different structures deriving from distinct functional components. These components (called ʻmetafunctionsʼ in systemic theory) are the ideational (clause as representation), the interpersonal (clause as exchange) and the textual (clause as message).
- Halliday (1985), a pag.158

[...] The fundamental components of 'meaning' in language are functional components. All languages are organized around two main kinds of meaning, the ʻideationalʼ or reflective, and the ʻinterpersonalʼ or active. These components, called ʻmetafunctionsʼ in the terminology of the present theory, are the manifestations in linguistic system of the two very general purposes which underlie all uses of language: (i) to understand the environment (ideational), and (ii) to act on the others in it (interpersonal). Combined with these is a third metafunctional component the ʻtextualʼ which breathes relevance into the other two.
- Halliday (1985), a pag.XIII