Halliday developed a theory of the fundamental functions of language, in which he analysed lexicogrammar into three broad metafunctions: ideational, interpersonal and textual. Each of the three metafunctions is about a different aspect of the world, and is concerned with a different mode of meaning of clauses. The ideational metafunction is about the natural world in the broadest sense, including our own consciousness, and is concerned with clauses as representations. The interpersonal metafunction is about the social world, especially the relationship between speaker and hearer, and is concerned with clauses as exchanges. The textual metafunction is about the verbal world, especially the flow of information in a text, and is concerned with clauses as messages. Malinowski's influence (see Figure 1.1) seems clear here: the ideational metafunction relates to the context of culture, the interpersonal metafunction relates to the context of situation, and the textual metafunction relates to the verbal context.
In each metafunction an analysis of a clause gives a different kind of structure composed from a different set of elements. In the ideational metafunction, a clause is analysed into Process, Participants and Circumstances, with different participant types for different process types (as in Case Grammar). In the interpersonal metafunction, a clause is analysed into Mood and Residue, with the mood element further analysed into Subject and Finite. In the textual metafunction, a clause is analysed into Theme and Rheme (as in the Prague School).
Figure 1.7, taken from [Matthiessen & Bateman 1991], shows an analysis of the sentence ``In this job, Anne, we're working with silver'' into three different structures in the three metafunctions. This kind of diagram is called a ``metafunctional layering'' diagram in SFG, but the metafunctions do not have any kind of relative ``depth'', rather they are different dimensions.
The metafunctional theory is part of the ``functional'' side of SFG, but it is also important in the ``systemic'' side of SFG. Each metafunction has a principal system in the networks for clauses, verbal groups and nominal groups. For example the TRANSITIVITY system is the principal system for the ideational metafunction in the clause network. These principal systems are shown in Figure 1.8, taken from [Matthiessen & Bateman 1991].
An important theoretical point is that in general, in the system networks, the systems within each metafunction are closely interconnected, but are largely independent of systems in the other metafunctions. System interconnections across metafunctions are rare. This is illustrated in Figure 1.9, taken from [Matthiessen & Halliday to appear].
In this network fragment, there are normal dependency relationships within the MOOD region of the interpersonal metafunction, between the MOOD-TYPE and INDICATIVE-TYPE systems and between the INDICATIVE-TYPE and INTERROGATIVE-TYPE systems, and there is also a further interconnection: the TAGGING system can be entered either from the imperative feature of the MOOD-TYPE system or from the declarative feature of the INDICATIVE-TYPE system. But there are no interconnections at all between the MOOD region of the interpersonal metafunction and the TRANSITIVITY region of the ideational metafunction.