Fawcett's Cognitive Model Of An Interactive Mind

Halliday & Matthiessen (1999: 428-9):

Within systemic-functional linguistics, Fawcett (e.g. 1980) has pioneered a "cognitive model of an interactive mind". There are many fundamental similarities with the approach we are taking here, e.g. in construing an experiential system of process configuration within the content plane. However, there are two related differences of particular interest in the context of our present discussion:

(i) in Fawcett's model, there is only one system-structure cycle within the content plane: systems are interpreted as the semantics, linked through a "realisation component" to [content] form, which includes items and syntax, the latter being modelled structurally but not systemically;

(ii) in Fawcett's model, the semantics is separate from the "knowledge of the universe", with the latter as a "component" outside the linguistic system including "long term memory" and "short term sort of knowledge".

Blogger Comments:

This is misleading, because 'form' in Fawcett's model also includes expression. Fawcett (2010: 35n, 39):

I take a different view, as this book shows, in that I regard the level of meanings within the 'lexicogrammar' as the key level of linguistically-realised meaning, such that it is realised in any one of (1) syntax, (2) intonation or punctuation (depending on the medium of discourse) and (3) items. …

The term "form" is used here in a wider sense than that in "Categories" (or indeed any of Halliday's later writings) because it includes, as well as syntax and grammatical and lexical items, components for intonation or punctuation (depending on whether the medium is speech or writing).

Lakoff's Cognitive Semantics vs A Social Semiotic Perspective

Halliday & Matthiessen (1999: 428):

Thus while our view that the ideational semantic system construes human experience is similar to what Lakoff (1987; 1988) calls the position of "experientialist cognition" (the position he has himself espoused, in contrast with what he calls "objectivist cognition"), it differs in that for us construing experience is an intersubjective process. It is at once both semiotic (the construction of meaning) and social (as in Peter Berger's "social construction of reality": cf. Berger & Luckmann, 1966, Wuthnow et al, 1984). It is the intersection of these two perspectives that characterises the social semiotic we are attempting to present in this book (cf. Lemke, 1995; Thibault, 1993).

Jackendoff's Conceptual Semantics vs A Socio-Semiotic Perspective

Halliday & Matthiessen (1999: 428):

Returning to Barwise's contrast between the mind-oriented view of meaning and the world-oriented view, we can note Barwise's general argument against the mind-oriented view:

... representational mental states have meaning in exactly the same way that sentences and texts have meaning, and saying what one means is a complicated matter. This makes attempts to explicate linguistic meaning in terms of mental representations an evasion of the main issue: How do meaningful representations of all kinds, sentences and states, mean what they do? (Barwise, 1988: 38)

We acknowledge this problem, but we believe the solution lies in a socio-semiotic view of meaning such as the one we are presenting here. Jackendoff views information about the projected world in conceptual terms; hence reality construction is seen as a process taking place within the consciousness of the individual. 

Our own view, that the projected world is a semantic construction, foregrounds the interpersonal perspective: meaning is construed in collaboration. Meanings are exchanged; and the "projected world" is constantly calibrated against the interpersonal negotiation of meaning. This means that consensus and conflict take over much of the domain that is usually conceptualised in terms of truth and falsehood (cf. Eggins, 1990). The semantic system (as part of the linguistic system) is shared; it is part of our social being.

Some Limitations Of Jackendoff's Ontology

Halliday & Matthiessen (1999: 427-8):

Jackendoff contrasts his ontology with that of "standard first-order logic", and makes the important point that the ontological classes of logic are vastly under-differentiated from a linguistic point of view — that this type of logic is not an adequate theory of the semantic structure of natural language. Compared to this type of logic, Jackendoffs ontology is much more highly differentiated However, it is not very rich compared to what we believe is needed in an account of the ideation base.

The classes recognised are roughly a list of semantic correlates of word classes at primary or secondary delicacy (such as one finds in traditional grammars). The list is not exhaustive, it does not include any significant paradigmatic organisation (i.e., it contains no organisation showing how types are arranged into a subsumption lattice) and some of the most revealing distinctions of the ideational semantics of English are absent — e.g. the distinction between phenomena and metaphenomena, the recognition of the role played by projection, and the expansion of the system through grammatical metaphor.

These are general observations. Since Jackendoff relies on reference as a source of evidence for the ontological distinctions, he might in fact have taken note of the semantically crucial phenomena of 'extended reference' (the move to 'macro') and reference to fact (the move to 'meta') discussed in Halliday & Hasan (1976).

Jackendoff's Ontology

Halliday & Matthiessen (1999: 427):

Jackendoff takes seriously the relation between conceptual organisation and syntactic organisation; this might be challenged from a classical formalist point of view, but from a functional point of view it is quite natural. In particular, he identifies correspondences between syntactic classes (i.e., categories in generative terms) and conceptual ones. Such correspondence is in fact a major source of evidence for the conceptual ontology. 

In particular, Jackendoff uses wh-items and non-interrogative reference items to support the ontology; he recognises things, amounts, places, directions, manners, events and actions. For instance, both things and places have to be recognised because English has both the forms what did you buy? and where is my coat? 

The ontology is tabulated in Table 10(1) below together with the grammatical evidence for each type. The left-most column provides a rough translation into our ideation base ontology.

The Transcendent Orientation Of Conceptual And Cognitive Semantics

Halliday & Matthiessen (1999: 426):

Jackendoff and a number of others now prefer the second position [semantic structures as subset of conceptual structures]. It is also shared by e.g. Langacker (1987), representing cognitive semantics from the other US coastline:

Meaning is a mental phenomenon that must eventually be described with reference to cognitive processing. I therefore side with Chafe (1970, p. 74-76) by adopting a "conceptual" or "ideational" view of meaning ... I assume it is possible at least in theory (if not yet in practice) to describe in a principled, coherent, and explicit manner the internal structure of such phenomena as thoughts, concepts, perceptions, images and mental experience in general. The term conceptual structure will be applied indiscriminately to any such entity, whether linguistic or nonlinguistic. A semantic structure is then defined as a conceptual structure that functions as the semantic pole of a linguistic expression. Hence semantic structures are regarded as conceptualisations shaped for symbolic purposes according to the dictates of linguistic convention, (pp. 97-8)

From our standpoint, this appears as a transcendent interpretation of meaning: we on the other hand prefer an immanent approach to meaning, where "conceptual organisation" is interpreted as meaning that is created by various semiotic systems, among which language is the primary one.

The Relation Of Conceptual To Semantic Organisation In Conceptual Semantics

Halliday & Matthiessen (1999: 425-6):

Jackendoff (1983) presents a major study of semantics and cognition from a generativist point of view. As part of that study, he presents an ontology of conceptual types that are linguistically motivated. While his purpose is theoretical rather than descriptive, and the ontology is not very extensive, it has become a frame of reference for work in this area.

Jackendoff sees semantic organisation as part of conceptual organisation — that part which can be verbalised; this is a position that distinguishes him from a number of other generativists. He identifies two possible positions on the relationship between semantic organisation and conceptual organisation (1983: Section 1.7; interpreted by us in Figure 10-3):

(1) "conceptual structure could be a further level beyond semantic structure, related to it by a rule component, often called pragmatics, that specifies the relation of linguistic meaning to discourse and to extralinguistic setting."

(2) "semantic structures could be simply a subset of conceptual structures — just those conceptual structures that happen to be verbally expressible".

Cognitive Semantics: Lakoff, Langacker, Johnson, Chafe, Talmy

Halliday & Matthiessen (1999: 425):

On the West Coast, a number of linguists have developed a "cognitive" alternative to generative linguistics. Some of them (e.g., Lakoff, Langacker) come from a generative background (Lakoff s starting point was generative semantics), but have made a radical departure from this tradition. They have widened the scope of study relative to the generativist research agenda so as to include metaphor as a prominent feature (Lakoff & Johnson, 1980, and other subsequent writings, such as Lakoff, 1987, 1988) and a detailed theoretical model of the relationship of language to cognition and perception (Langacker, 1987). A few have also oriented their work towards discourse (notably Chafe, e.g. 1979; 1987; cf. also Tomlin's, 1987a, discussion of the linguistic reflection of cognitive events). 

This version of cognitive semantics is arguably more closely associated with the rhetorical and ethnographic tradition (perhaps not so much in terms of its roots, but in terms of where it is headed); cognitive anthropology, with its interest in folk taxonomy and more recently in cultural models, provides a meeting point between the two.

Various aspects of the West Coast work in cognitive semantics are relevant to the organisation of the ideation base; for example, the work on metaphorical systems already mentioned, Talmy's (e.g. 1985) work on lexicalisation, and Chafe's (1970) early work on the organisation of meaning.

Conceptual Semantics: Jackendoff

Halliday & Matthiessen (1999: 424-5, 424n):

On the East Coast, Jackendoff (e.g., 1983), drawing on his earlier work on interpretive semantics (Jackendoff, 1972), has developed a generativist type of cognitive semantics,⁴ which he calls conceptual semantics. This is integrated with a number of aspects of modem generative theory, e.g. the X-bar syntactic subtheory. Jackendoff puts forward a conceptual ontology and suggestions for conceptual structure that we shall return to below. This version of cognitive semantics is arguably the more closely associated with the logical and philosophical tradition (cf. Jackendoff, 1988: 81-2).

 

⁴ Jackendoff calls his approach conceptual semantics and Lakoff calls his cognitive semantics. We use the term cognitive semantics as the generic term for cognitively oriented approaches to semantics.

(Mind-Oriented) Cognitive Semantics

Halliday & Matthiessen (1999: 424):

Cognitive semantics has emerged out of the generative tradition in the last ten to fifteen years. It is a natural development of the general cognitivist research program that began in the 1950s with the birth of cognitive science, Artificial Intelligence, and Chomsky's cognitively oriented linguistics.

Those adopting the cognitive approach to semantics share certain assumptions about semantic organisation as part of conceptual organisation, and tend to reject formal, Montague-style semantics, as indicated by the passages from Jackendoff and Lakoff quoted above. However, they seem to fall into two groupings, which can be conveniently described in terms of the US coastline [east vs west].

The Constructivist View Of Reality In SFL Theory

Halliday & Matthiessen (1999: 423-4):

We also have emphasised that reality is not something that is given to us; we have to construct an interpretation of it — or, as we prefer to put it, we have to construe our experience. Interpretation is a semiotic process, and our interpretation takes into account not only the concrete natural world but also the socio-cultural realm that is brought into existence as a semiotic construct (see Hasan, 1984a, for discussion, with reference to Whorf).

Blogger Comments:

Strictly speaking, in this view, 'reality' is the interpretation, not what is interpreted, and the 'concrete natural world' is also a semiotic construct: a construal of experience as meaning: perceptual and linguistic.

(World-Oriented) Formal Semantics Viewed From (Mind-Oriented) Cognitive Semantics: Lakoff

Halliday & Matthiessen (1999: 423):

Writing from a different orientation within cognitive semantics, Lakoff (e.g. 1988) is also very critical of what he calls "objectivist metaphysics" (metaphysical realism). He presents a detailed critique of this position, also noting the problem that arises if meanings are located in the world:

To view meaning as residing only in the relationship between symbols and external reality is to make the implicit claim that neither colour categories, nor any other secondary category, should exist as meaningful cognitive categories. Yet colour categories are real categories of the mind. They are meaningful, they are used in reason, and their meaning must be accounted for. But the mechanism of objectivist cognition cannot be changed to accommodate them without giving up on the symbolic category of meaning. But to do that is to abandon the heart of the objectivist program, (p. 132)

(World-Oriented) Formal Semantics Viewed From (Mind-Oriented) Cognitive Semantics: Jackendoff

Halliday & Matthiessen (1999: 423):

Jackendoff (1983: Ch. 2), working within cognitive semantics, argues that the account of 'aboutness' in formal semantics is simplistic:

... I will take issue with the naive (and nearly universally accepted) answer that the information language conveys is about the real world, (p. 24) ... If indeed the world as experienced owes so much to mental processes of organisation, it is crucial for a psychological theory to distinguish carefully between the source of environmental input and the world as experienced. For convenience, I will call the former the real world and the latter the projected world (experience world or phenomenal world would also be appropriate), (p. 28) ...  

It should now be clear why we must take issue with the naive position that the information conveyed by language is about the real world. We have conscious access only to the projected world — the world as unconsciously organised by the mind; and we can talk about things only insofar as they have achieved mental representation through these processes of organisation. Hence the information conveyed by language must be about the projected world. We must explain the naive position as a consequence of our being constituted to treat the projected world as reality.

According to this view, the real world plays only an indirect role in language: it serves as one kind of fodder for the organising processes that give rise to the projected world. If this is the case, we must question the centrality to natural language semantics of the notions of truth and reference as traditionally conceived. Truth is generally regarded as a relationship between a certain subset of sentences (the true ones) and the real world; reference is regarded as a relationship between expressions in a language and things in the real world that these expressions refer to. Having rejected the direct connection of the real world to language, we should not take these notions as starting points for a theory of meaning. Thus an approach such as that of Davidson (1970), which attempts to explicate natural language semantics in terms of Tarskian recursive theory of truth, is antithetical to our own inquiry, (pp. 29-30)

The projected world in Jackendoff's account is the result of creative acts of perception: it is constructed as a model of sensory input, but with the significant addition of information from the perceptual system itself.

(World-Oriented) Formal Semantics Viewed From (Mind-Oriented) Cognitive Science

Halliday & Matthiessen (1999: 422-3):

It would seem that formal semantics is quite far from the concerns of cognitive science; but formal semantics is often carried out within the broad program that cognitive scientists adopt. Writing from a cognitive standpoint, Johnson-Laird (1983) assesses model-theoretic semantics as follows:

The power of model-theoretic semantics resides in its explicit and rigorous approach to the composition of meanings. ... Some idealisations definitely complicate the 'ecumenical' use of model-theoretic semantics as a guide to psychological semantics. One such idealisation is the mapping of language to model without any reference to the human mind, and this omission gives rise to certain intractable difficulties with the semantics of sentences about beliefs and other such prepositional attitudes. These difficulties are readily resolved within the framework of mental models, (p. 180)

... model-theoretic semantics should specify what is computed in understanding a sentence, and psychological semantics should specify how it is computed, (p. 167)

Johnson-Laird's own psychological approach is one based on mental models.

Formal Semantics: The Model Theoretic Approach To Meaning

Halliday & Matthiessen (1999: 422):

The model theoretic approach to meaning is a transcendent one: meanings are located in the world outside of language. This is seen as a strength by the proponents of the model theoretic approach. Dowty et al (1981) criticise various other theories of meaning for ignoring the "inherent aboutness of language", its relationship to the real world:

Any theory which ignores this central property, it is argued, cannot be an adequate theory of natural language. Examples would be theories which, in effect, give the meaning of a sentence by translating it into another language, such as a system of semantic markers or some son of formal logic, where this language is not further interpreted by specifying its connection to the world. The approach of Katz and his coworkers seems to be of this sort (Katz and Fodor, 1963; Katz & Postal, 1964), as is that of Jackendoff (1972) and of the framework of Generative Semantics (Lakoff, 1972; McCawley, 1973; Postal, 1970).

Formal Semantics: Meaning As Extensional And Intensional

Halliday & Matthiessen (1999: 422):

In modelling meaning, formal semantics models it as both extensional and intensional: extensional meaning is cast in terms of sets of entities in a model of some world, and intensional meaning is the relation between a linguistic expression and its extension in any possible world. Formal semantics has thus been concerned with how semantic representations can be related to some extra-linguistic world model. Dowty et al (1981: 5) emphasise the relationship between language and the world in their introductory characterisation of truth conditional semantics:

"... truth conditional semantics, in contrast to other approaches mentioned [including Katz & Fodor, Jackendoff, and generative semantics, MAKH & CM], is based squarely on the assumption that the proper business of semantics is to specify how language connects with the world — in other words, to explicate the inherent "aboutness" of language."

This is, of course, an ideational orientation: the focus here is on representational meaning only. One central method has been to build models of the world in set theoretic terms, and to relate these to linguistic expressions — the model theoretic approach.

Formal Semantics As Syntagmatically Organised

Halliday & Matthiessen (1999: 422):

In its relation to syntax, the formal approach to semantics is compositional, being formulated in terms of syntagmatic organisation: the meaning of larger syntagmatic units such as sentences is built up as functions of smaller units such as nouns and verbs. This semantic composition is in tandem with the syntax, so what becomes important is the nature of the syntactic rules themselves, not just the result of their application. Montague used a form of categorial grammar in formalising the rules of syntax for this purpose.

The World-Oriented Tradition: The Origin Of Formal Semantics

Halliday & Matthiessen (1999: 419):

The development of logic is of interest in itself in the present context as a tradition of 'semiotic design' whereby logical systems were developed out of the resources that have evolved in language. This design of logical systems was earned out to meet certain restricted purposes having to do with reasoning; but once such systems had been developed, philosophers and linguists turned them back on language and used them as systems for representing and theorising meaning. In the present century, when formal semantics was developed for underpinning logical systems, this was then taken as a model for work on the semantics of natural languages.

Important Developments In The Logico-Philosophical Tradition

 Halliday & Matthiessen (1999: 419, 420):

Here we are merely noting some particularly important developments. These are charted in Figure 10-2.

The Logico-Philosophical vs Rhetorical-Ethnographic Orientations To Meaning Summarised

Halliday & Matthiessen (1999: 418):

The two different orientations are summarised with the internal differences we have just reviewed in Figure 10-1.

The Semantic Focus Of The Logico-Philosophical vs Rhetorical-Ethnographic Orientations

Halliday & Matthiessen (1999: 417-8):

…we can note that the two orientations differ in what kind of semantic organisation they focus on. 

In the logico-philosophical orientation scholars have focussed on syntagmatic organisation: they have been concerned with semantic structure — including principles relating to structure such as those of compositionality and semantic decomposition. For example, in their analysis of the senses of "words", they have tended to analyse these as being composed of semantic components, semantic markers, semantic primitives or the like (following Katz & Fodor, 1963, in the generative tradition). 

In the rhetorical-ethnographic orientation scholars have focussed on both paradigmatic and syntagmatic organisation, often foregrounding the paradigmatic: they have been concerned with the semantic system, with the meaning potential — including principles of taxonomy and the metafunctional simultaneity of systems. For example, scholars in "ethnoscience" have studied folk taxonomies of animals, plants, diseases and the like; and systemic-functional scholars have tried to map out semantic systems such as those of speech function and conjunction. Another example is our own work on the ideation base presented here.

The Metafunctional Scope Of The Logico-Philosophical vs Rhetorical-Ethnographic Orientations

Halliday & Matthiessen (1999: 417):

The two orientations differ in the metafunctional scope of their models of semantics. 

In the logico-philosophical orientation, meaning is closely associated with representation, reference, denotation, extension or 'aboutness', so the metafunctional scope is restricted to the ideational metafunction: semantics means ideational semantics. 

In the rhetorical-ethnographic orientation, meaning is closely associated with rhetorical concerns, so the metafunctional scope involves all three metafunctions: semantics means ideational, interpersonal and textual semantics; it is multifunctional. 

If interpersonal and textual meanings are dealt with by logico-philosophical accounts (they are often outside their scope), they are handled under the heading of pragmatics rather than the heading of semantics. 

For example, speech act theory was developed as a logico-philosophical interpretation of speech function (or rather of its ideational construal) and has come to be included within pragmatics.

The Basic Unit Of Meaning In The Logico-Philosophical vs Rhetorical-Ethnographic Orientations

Halliday & Matthiessen (1999: 417):

The approaches differ with respect to what they take as the basic unit of meaning. 

In the logico-philosophical orientation, the basic unit tends to be determined "from below", from grammar: since sentences are seen as encoding propositions, the basic unit of semantics is the proposition (as in propositional calculus).

In contrast, in the rhetorical-ethnographic orientation, the basic unit tends to be determined "from above", from context: since language is seen as functioning in context, the basic unit of semantic is the text (see Halliday & Hasan, 1976; Halliday, 1978a). 

So in the logico-philosophical orientation, semantics means in the first instance propositional semantics, whereas in the other orientation it means text semantics.

The Transcendent View Of Meaning: World-Oriented vs Mind-Oriented

Halliday & Matthiessen (1999: 416-7):

The modern split within the "transcendent" view is between what Barwise (1988: 23) calls the world-oriented tradition and the mind-oriented tradition, which he interprets as public vs. private accounts of meaning:

The world oriented tradition in semantics, from Tarski on, has focussed on the public aspect of meaning by trying to identify the meaning of a sentence or text with its truth conditions, the conditions on the actual world that are needed to insure its truth (Davidson 1967 and Montague 1974).  

The psychological tradition, by contrast, has focussed on the private aspect by trying to identify meaning with an intrinsically meaningful mental representation (Fodor 1975 or Jackendoff 1983 e.g.).

The world-oriented tradition interprets meaning by reference to (models of) the world; for example, the meaning of a proper noun would be an individual in the world, whereas the meaning of an intransitive verb such as run would be a set of individuals (e.g. the set of individuals engaged in the act of running). 

The mind-oriented tradition interprets meaning by reference to the mind; typically semantics is interpreted as that part of the cognitive system that can be "verbalised".

Meaning Seen As Transcendent

Halliday & Matthiessen (1999: 416):

Many traditional notions of meaning are of the second kind — meaning as reference, meaning as idea or concept, meaning as image. These notions have in common that they are 'external' conceptions of meaning; instead of accounting for meaning in terms of a stratum within language, they interpret it in terms of some system outside of language, either the 'real world' or another semiotic system such as that of imagery.

The Location Of Meaning In The Logico-Philosophical vs Rhetorical-Ethnographic Traditions

 Halliday & Matthiessen (1999: 416):

The two orientations towards meaning thus differ externally in what disciplines they recognise as models. These external differences are associated with internal differences as well.

First, the orientations differ with respect to where they locate meaning in relation to the stratal interpretation of language:

(a) intra-stratal: meaning is seen as immanent — something that is constructed in, and so is part of, language itself. The immanent interpretation of meaning is characteristic of the rhetorical-ethnographic orientation, including our own approach.

(b) extra-stratal: meaning is seen as transcendent — something that lies outside the limits of language. The transcendent interpretation of meaning is characteristic of the logico-philosophical orientation.

The Logico-Philosophical vs Rhetorical-Ethnographic Orientations in Western Thinking About Meaning

Halliday & Matthiessen (1999: 415-6):

We can identify two main traditions in Western thinking about meaning (see Halliday, 1977):

(i) one oriented towards logic and philosophy, with language seen as a system of rules;

(ii) one oriented towards rhetoric and ethnography, with language seen as resource.

It is typically the logical-philosophical tradition that provides the background for work on knowledge representation and proposals for the knowledge base. Since the 50s, a link has been forged between this tradition and cognitivism under the general rubric of cognitive science. 

However, although it is less often referred to, the rhetorical-ethnographic tradition is equally relevant to work on the modelling and representation of knowledge — for example, in recent times, the work on folk taxonomy carried out by anthropologists and anthropological linguists (often in the framework of ethnoscience) and the work on intellectualisation in the Prague School framework are central to understanding the organisation of the ideation base. 

Our own work here falls mainly within the second tradition — but we have taken account of the first tradition, and the general intellectual environment in which versions of our meaning base are being used also derives primarily from the first tradition. Indeed, the two traditions can in many respects be seen as complementary, as contributing different aspects of the overall picture. Our own foundation, however, is functional.

Mainstream work in psycholinguistics, e.g. Miller & Johnson-Laird (1976), is in general located within the logico-philosophical tradition. Two other distinctive contributions to the study of meaning within linguistics, those of Lamb (1964, 1992) and of Wierzbicka (1980, 1988), derive in more or less equal measure from both the major intellectual movements we have referred to.

The two orientations towards meaning thus differ externally in what disciplines they recognise as models.

Example Of The Discourse Function Of Ideational Metaphor

Halliday & Matthiessen (1999: 402-3):

In the second type [of ideational metaphor] to be exemplified, a clause or clause complex is nominalised as a Medium participant in a clause whose Process simply means 'happen' (i.e., existence of a process: happen, occur, take place; begin, continue, stop). The Medium constitutes a thematic quantum of information, as in the following example:

However, Einstein realised that if an observer was travelling alongside a light wave at the same speed as the light wave, the wave would essentially disappear, as no wave peaks or troughs would pass by the observer. But the disappearance of light waves because of the motion of an observer should not happen according to Maxwell, so Einstein concluded that either Maxwell's equations were wrong or that no observer could move at the speed of light.

Here the metaphorical clause makes it possible to summarise the preceding clause complex the wave would essentially disappear, as no wave peaks or troughs would pass by the observer as a thematic nominalised participant, the disappearance of light waves because of the motion of an observer.

The Textual Motivation Of Metaphorical Mental Clauses Of Perception

Halliday & Matthiessen (1999: 401-2):

Metaphorical mental clauses of perception, such as the fifth day saw them at the summit (cf. Halliday, 1985: 324-5), may serve to create two quanta of information, Senser (Theme) + Phenomenon (New); consider the following example:

Summer finds campers and hikers descending on the mountains in throngs

Here the metaphorical clause summer finds campers and hikers descending on the mountains in throngs is textually motivated in terms of both THEME and INFORMATION. From the point of view of THEME, it provides summer as the unmarked Theme of the clause — one instalment in the method of development. From the point of view of INFORMATION, it groups campers and hikers descending on the mountains in throngs as one quantum of information (the Phenomenon of the metaphorical mental clause); contrast the congruent version, which has the same Theme (marked) but a different culmination of the New: 

In summer, campers and hikers descend on the mountains in throngs.

The Discourse Function Of Ideational Grammatical Metaphors

Halliday & Matthiessen (1999: 401):

These two clause systems, THEME IDENTIFICATION and THEME PREDICATION, are components of the overall system of THEME in English grammar. They produce structures in which identifying relational processes are used to reconstrue figures as equations (the two together are referred to as "thematic equatives" in Halliday, 1985: 41-4). But ideational grammatical metaphors typically have a discourse function of this kind; they are as it were pressed into service by the textual metafunction, to provide alternative groupings of quanta of information.