Directionality in Discourse: Prominence Differences in Subordination Relations1

This paper proposes a new approach to discourse directionality, a phenomenon which, as is well known, is neither well defined nor adequately accounted for. Directionality is the property of (a part of) a discourse to be directed towards a 'goal', usually implying asymmetric functional relations between the discourse units involved. The direction of such an asymmetric discourse relation depends on whether the unit that provides the goalsatisfying value precedes or follows the unit which is subservient to it. Fundamental to our proposal is an analysis of directionality in terms of the topic-comment distinction. Within this framework, directionality is defined as a recursive property assigned to higher-order and lower-order discourse relations central to which is the assumption that they are realized by explicit or implicit topic-forming questions. It will be shown that the distinction that can be made between three types of directionality is precisely a function of three different ways of quantitative/qualitative subordination realized by subquestioning. Apart from the resulting theory providing a solution to the definition problem, it also provides an answer to the determination problem which implies that we attribute a criterion to distinguish dominant discourse units from subservient ones. In addition, the theory contributes to the much discussed issue of an adequate formalization of those discourse elaboration processes that do not involve a new partial value but merely support an already introduced 'subject matter'. 1 I N T R O D U C T I O N If one is confronted with the task of structuring an extended answer to a question, there are, in principle, three options: first, operating towards the 'goal' of the question implying the so-called goal-subservient part to precede the part providing the goal-satisfying value; second, acting inversely, by firstly specifying that which is ultimately asked for and then elaborating on it for supportive reasons, e.g. providing a justification of the answer value given; and third, acting as it were in both directions, implying that the discourse units comprising the extended answer are mutually subservient in satisfying the goal of the question. However, it is generally assumed (especially Allen 1983; Allen & Perrault 1980; Grosz & Sidner 1986; Litman 1985; Mann & Thompson 1988; Moore & Pollack 1992; Pollack 1986; Wilensky 1983) that these options apply not only to the structure of complex answers to explicit questions but also to the structure of extended discourse in general. 364 Directionality in Discourse The phenomenon under discussion is that of discourse directionality, a phenomenon that is central in, in particular, the rhetorical and intentional approaches of discourse structure. Directionality refers to the property of (a part of) a text to be directed towards a 'goal' or 'point', usually resulting in asymmetric functional relations between discourse segments. This asymmetry implies a division of (that part of) the text into two related segments, one of which is subservient to the other, with the latter being identical to the part providing the goal-satisfying value or a part of this value. Various terms are used to refer to the phenomenon of directionality. A frequently used term is nuclearity in Rhetorical Structure Theory (RST; e.g. Mann & Thompson 1988). In RST, nuclearity is considered to be 'a central organizing principle of text structure' (1988: 267), referring to a functional distinction applicable to rhetorical relations between discourse units ('spans'), namely that one of them, the nucleus, is functionally dominant while the other related unit, the satellite, is subservient. It is assumed that in most cases a functional asymmetry such as this is present in, e.g., 'If A is evidence for B, then B is not evidence for A' (1988: 266). Another term commonly used in the intentional approaches of discourse structure is the term dominance (Grosz & Sidner 1986). The term refers to one of the characteristics of intentional relations between discourse segments, namely that if one discourse segment purpose DSP, dominates another discourse segment purpose DSP2, DSP2 contributes to DSP, (1986: 179). As made clear in Moser & Moore (1993), dominance relations such as these correspond to the direction of intentional relations in RST, i.e. 'the satellite span, S, affects the purpose of the nucleus span, N, only if the intention that S realizes is dominated by the intention that S and N (and possibly others) realize together' (1993: 95). However, in spite of the central role of directionality in theories of discourse structure, we are nevertheless confronted with both a definition and a determination problem. The former is probably the most serious one and implies the absence of a fully adequate formal definition. For instance, RST, in which the notion of directionality is most central, does not provide so much a definition as a taxonomy of characteristic properties of this notion. The characterizations given are, among others, that the dominant discourse unit, the nucleus, is 'more essential to the writer's purpose', is 'more central', is 'more deserving of response, including attention, deliberation and reaction'. Although many of these characterizations present relevant descriptions of the phenomenon, they do not show the kind of phenomenon with which we are precisely dealing. In particular, they lack a full explanation of why the non-dominant part, the satellite, may be deleted while preserving the coherence and intention of the text. In addition, they Jan van Kuppevelt 365 leave unexplained why the dominant part is functionally more prominent, i.e. more deserving of attention, deliberation, and reaction than the functionally less prominent part. Obviously, if a definition is to be considered adequate, it must provide satisfactory answers to these questions. Closely related to the definition problem is the task of determining the relevant factor stating which of two related discourse units is functionally dominant. It is generally assumed that in the case of functional asymmetric discourse relations the order of dominant and non-dominant discourse units is not directly influenced by the discourse relation between them. The literature also fails to provide an adequate solution to this problem. In addition to the definition problem and the determination problem, one more issue, though a less central one, has to be mentioned, namel aie formalization of non-dominant, supportive discourse units. Usually a conquerable part of discourse consists of justifications, motivations, etc. which in themselves do not introduce a 'subject matter' or 'independent value', but have a supportive function with respect to the dominant discourse parts which provide such a value. How can the effect of this supportive material be formalized in terms of, e.g., set theory, if the function of this material is merely a secondary, not providing independent values as such? We will propose an account of directionality which provides an adequate definition of the phenomenon including a corresponding criterion for identifying functionally dominant and non-dominant discourse units. In addition, the proposal provides the formalization referred to above. Essential to the proposal is an account of directionality in terms of the topic-comment distinction which, by definition, applies, in a uniform way, to individual utterances and larger discourse units. It is demonstrated how the phenomenon of directionality is determined by the discourse-internal topic-comment structure. Fundamental to the proposal is the underlying hypothesis that the topiccomment structure of discourse results from the process of answering higher-order and lower-order explicit and implicit questions in discourse. This necessarily implies a distinction between the main structure of discourse and its embedded, lower-order subordinate structures. As we will see, the former is analysed as an answer to a (set of) leading higherorder, topic-forming question(s) defining the global discourse topic, while the latter results from subquestions. By definition, substructures are hierarchically embedded in the main structure of discourse because of the specific completion function of the associated subquestions, namely to contribute to the unsatisfactory answer given to the overall, discourse-topic defining question. 366 Directionality in Discourse Directionality is then characterized as a property assigned to a discourse relation between two discourse units. According to the framework this relation is realized by an explicit or implicit topic-forming question. It will be demonstrated that dominant discourse units providing goal-satisfying values are not determined by such a question but rather by the higher-order question defining the common topic of both the dominant and the related non-dominant discourse unit. In Section 2, we briefly illustrate the different types of directionality that can be distinguished, and include an illustration of the fact that these types may occur on different structural levels, corresponding to the discourseinternal topic-comment structure. In Section 3, an account is given of this mostly hierarchical structure which, as indicated above, results from the process of the contextual induction of explicit and implicit topic-forming questions. Finally, Section 4 demonstrates how the different types of directionality are determined by this topic-comment structure. 2 THREE TYPES OF DIRECTIONALITY 2.1 Description of the phenomenon In the introduction we have informally characterized the phenomenon of discourse directionality as the property of a (part of a) text to be directed towards a 'goal'. Usually this implies a division of this (part of the) text into two related segments, one of which is functionally less prominent. In the intentional approaches of discourse, this so-called non-dominant discourse unit is analysed as that part of the relation which is subservient to the discourse purpose associated with both discourse units, leaving unspecified the exact (intentional) status of the related dominant discourse unit. In this paper a distinction is made between the following three types of directionality: Forward (FW) Directionality, Backward (BW) Directionality, and Bi-Directionality. We will demonstrate (Section 4) the specific way in which these types correspond to three different types of subordination relations in discourse. In all cases of directionality the phenomenon applies to a succession of two related discourse units. We present the following description: FW Directionality, BW Directionality, and Bi-Directionality Given two related discourse units Us and U;,,,, the discourse relation R between them, R(Uj, Ui*n), is characterized by FW Directionality if U-, is subservient to the succeeding discourse unit U,*,,, i.e. the discourse unit that (in a sense which has to be explicated) provides the goal-satisfying value in this case, by BW Directionality if the reverse applies, and by Bi-Directionality if such a functional asymmetry is absent. Jan van Kuppevelt 367 The absence of asymmetric functional relations in case of Bi-Directionality implies that the related discourse units U; and U i + n are equally prominent and that together they contribute to the goal of the text segment which comprises the two of them.' One of the central hypotheses of the framework to be presented is that in this context every discourse relation R(Uj, U i+n) is determined by an explicit or implicit question introducing the topic of the (extended) discourse unit answering this question. By definition, such a topic-forming question is induced as the result of the discourse unit Uj and is answered by the succeeding discourse unit U i + n . In anticipation of a further explication of this hypothesis, we will illustrate the different types of directionality in terms of discourse relations realized by such questions. An illustration of FW Directionality is given in (1), Two implicit questions are added to the original text, providing an analysis of the structure of this text in terms of questions and answers. Angled brackets indicate the implicit character of a question; arrows indicate the direction of a discourse relation. (1) At this telephone company we assume you enjoy a