Parsimonious and Profligate Approaches to the Question of Discourse Structure Relations

To computationalists investigating the structure of coherent discourse, the following questions have become increasingly important over the past few years: Can one describe the structure of discourse using interclausal relations? If so, what interclausal relations are there? How many are required? A fair amount of controversy exists, ranging from the parsimonious position (that two intentional relations suffice) to the profligate position (that an open-ended set of semantic/rhetorical relations is required). This paper outlines the arguments and then summarizes a survey of the conclusions of approximately 25 researchers from linguists to computational linguists to philosophers to Artificial Intelligence workers. It classifies the more than 350 relations they have proposed into a hierarchy of increasingly semantic relations, and argues that though the hierarchy is open-ended in one dimension, it is bounded in the other and therefore does not give rise to anarchy. Evidence for the hierarchy is mentioned, and its relations (which are rhetorical and semantic in nature) are shown to be complementary to the two intentional relations proposed by the parsimonious position. H o w M a n y I n t e r c l a u s a l Disc o u r s e C o h e r e n c e R e l a t i o n s ? This paper proposes an answer to an issue that keeps surfacing in the computational study of the nature of multisentential discourse. It has been argued fairly generally that multisentence texts (specifically, short texts such *This work was supported by the Rome Air Development Center under RADC contract FQ7619-89-03326-0001. as paragraphs) are coherent by virtue of the rhetorical or semantic relationships that hold among individual clauses or groups of clauses (see, for example, [Aristotle 54, Hobbs 79, Grimes 75, Mann & Thompson 88]. In this view, a text is only coherent when the speaker aids the heater 's inferential understanding processes by providing clues, during the discourse, as to how the pieces of the text interrelate. Such clues are often cue words and phrases such as "in order to" (signalling a purpose for an action) or "then" (signalling the next entity in some temporal or spatial sequence); but they can also be shifts in tense and mode (such as in "She was gone. Had she been there, all would have been well"), and even appropriate pronominalizations. Various researchers in various intellectual subfields have produced lists of such relations for English. Typically, their lists contain between seven and thirty relations, though the more detailed the work (which frequently means the closer the work is to actual computer implementation), the more relations tend to be named. I have collected the lists of over 25 researchers from philosophers (e.g., [Toulmin 58]) to linguists (e.g., [Quirk & Greenbaum 73, Halliday 85]) to computational linguists (e.g., [Mann & Thompson 88, Hobbs 79]) to Artificial Intelligence researchers (e.g., [Schank & Abelson 77, Moore & Paris 89, Dahlgren 88]) amounting to a total of more than 350 relations. The researchers and their lists appear below. In this paper, I will call the assumption of these researchers, namely that some tens of interclausal relations are required to describe the structure of English discourse, the Profligate Position.