A Syntactic Approach To Discourse Semantics

A correct structural analysis of a discourse is a prerequisite for understanding it. This paper sketches the outline of a discourse grammar which acknowledges several different levels of structure. This gram~nar, the "Dynamic Discourse Model", uses an Augmented Transition Network parsing mechanism to build a representation of the semantics of a discourse in a stepwise fashion, from left to right, on the basis of the semantic representations of the individual clauses which constitute the discourse. The intermediate states of the parser model the intermediate states of the social situation which generates the discourse. The paper attempts to demonstrate that a discourse may indeed be viewed as constructed by means of sequencing and recursive nesting of discourse constituents. It gives rather detailed examples of discourse structures at various levels, and shows how these structures are described in the framework proposed here. "I DISCOURSE STRUCTURES AT DIFFERE.NT LEVELS If a discourse understanding system is to be able to assemble the meaning of a complex discourse fragment (such as a story or an elaborate description) out of the meanings of the utterances constituting the fragment, it needs a correct structural analysis of it. Such an analysis is also necessary to assign a correct semantic interpretation to clauses as they occur in the discourse; this is seen most easily in cases where this interpretation depends on phenomena such as the discourse scope of temporal and locative adverbials, the movement of the reference time in a narrative, or the interpretation of discourse anaphora. The Dynamic Discourse Model, outlined in this paper, is a discourse grammar under development which analyses the structure of a discourse in order to be able to deal adequately with its semantic aspects. It should be emphasized at the outset that this system is a formal model of discourse syntax and semantics, but not a computer implementation of such a model. For a system to be able to understand a discourse, it must be able to analyse it at several different levels. i. Any piece of talk must be assigned to one Interaction -i.e., to a socially constructed verbal exchange which has, at any moment, a well-defined set of participants. 2. Virtually every interaction is viewed by its participants as belonging to a particular predefined genre -be it a doctor-patient interaction, a religious ceremony, or a casual chat. Depending on the genre, certain participants may have specific roles in the verbal exchange, and there may be a predefined agenda specifying consecutive parts of the interaction. An interaction which is socially "interpreted" in such a fashion is called a Speech Event (Hymes,1967,1972). 3. A stretch of talk within one Speech Event may be characterized as dealing with one Topic. 4. Within a Topic, we may find one or more Discourse Units (DU's) -socially acknowledged units of talk which have a recognizable "point" or purpose, while at the same time displaying a specific syntactic/semantic structure. Clear examples are stories, procedures, descriptions, and jokes. 5. When consecutive clauses are combined into one syntactic~semantic unit, we call this unit a discourse constituent unit (dcu). Examples are: lists, narrative structures, and various binary structures ("A but B", "A because B", etc.). 6. Adjacency Structures may well be viewed as a kind of dcu, but they deserve special mention. They are two or three part conversational routines involving speaker change. The clearest examples are question-answer pairs and exchanges of greetings. 7. The smallest units which we shall deal with at the discourse level are clauses and operators. Operators include "connectors" like "and", "or", "because", as well as "discourse markers" like "well", "so", "incidentally". The levels of discourse structure just discussed are hierarchically ordered. For instance, any DU must be part of a Speech Event, while it must be built up out of dcu's. The levels may thus be viewed as an expansion of the familiar linguistic hierarchy of phoneme, morpheme, word and clause. This does not mean, however, that every discourse is to be analysed in terms of a five level tree structure, with levels corresponding to dcu, DU, Topic, Speech Event and Interaction. To be able to describe discourse as it actually occurs, discourse constituents of various types must be allowed to be embedded in constituents of the same and other types. We shall see various examples of this in later sections. It is worth emphasizing here already that "high level constituents" may be embedded in "low level constituents". For instance, a dcu may be interrupted by a clause which initiates another Interaction. Thus, a structural description of the unfolding discourse would include an Interaction as embedded in the dcu. In