Investigating Discourse Relations

Ill this paper, we present the theoretica.1 foun(la.tions which underlie our current research. We emphasize a methodologicaJ point: connections among proposed theories of discourse structure must be carefiflly explicated. By making these connections, we ca.n (letermilm whether theories ma.ke equiva.lent claims, consistent 1)ut unrelated cla.ims or differ iu substa.ntive issues. A synthesis of current theories is required, a descriptive fl'a.mework which assumes the common ground and which highlights issues to be resolved. Further, there a.re two distinct :lh.ctors to guide us ill developing such a synthesis. First, the descriptive fl'a.mework nmst facilita.te the fi:)rmula.tion of controversi;d issues in terms of empirical predictions. Then, the synthesis ca.n I)e developed a.nd refined by linguistic studies. Second, the fr~uuework must be a.pplicaJ)le to computer processing of na.tural language, both understa.nding and gener~tion. Our particula.r interest is the computer generation of explanations in a. tutoring system. Idea.lly, the fi'a.mework will provide a. me~ns through which the results of discourse a.nalysis and computatiorml linguistics ca.n inform one another. In its broadest outline, the goaJ is to uu(lersta.ad the precise i~H:era.ction between fea.tures of tbrm, meaning a.nd ffim(:tion ill the crea.tioll of discourse coherem:e. Wha.t kiuld o1' tbrm, mea.ning ~tnt(i flmction links occur ])etween uttera.Jlces a.nd how a.re these thre~, ki,l(Is of links recognized? In a tirst step towards the synthesis we wouhl like to see, we will discuss the l~erspective which the G&S (Grosz and Sidner 1986) and RST (=RhetoricaJ Structure Theory, Ma.nn and Thompson 1988) theories take on links of meaning a.nd function. We conclude with a brief description of a.n empiricaJ study suggested by this theory compa.rison. Note tha.t we consider only monologic discourse at this time, believing genera.lizz~tions between this a.nd multi-agent discourse to be prema.ture. In the study and discussion of rhetorica.l relations, the terminology ha.s become nonsta,ndard a.nd confusing. Here, we a.dol)t the term "discourse rela.tions" to mea.n all the conn(,ctions a.mong the exl)ressions ill (liscourse which, ta.keu tog(,th(~r, a.ccount 1or its ('()]lerence. So, in order to be a. (liscourse relation in the sense used here, two criteri,~ a.re requir(,(I. One, the rebttion concerns elements of a sentence utterance a.nd other utteraJtces in the context. Two, the rel;Ltion must be recognized in order to understand the discourse, i.e., it contril)utes to coherence ra.ther than another concern such as style. As suggested above, SOlne fea.ture of form, mea.ning or fimction defines a discourse relation. These will be termed textual, informa.tiona.1 a.nd intentionaJ discourse relations respectively. Informational a.nd intentional discourse rela.tions a.re essentia.lly non-linguistic in the sense that they do not originate with la.nguage. Mffch recent work on discourse relations either explicitly discusses or implictly uses a. distinction between in|brma.tionaJ and intentiona.1 relatious (Schiffrin 1987, Redeker 1990, Hovy and Ma.ier 1992, Moore a.nd Polla.ck 1992, Moser 1992, Sanders, Spooren and Noor(hnan 1992, inter aJia.). The distinction, a. kind of sema.ntic-l)ragmatic distinction, concerns the source of discourse rela,tions, whether ,~ rela.tion orgina,tes with what is being talked about (informationa.1) or with why we a.re taJking about it (intentionaJ). Inibrma.tionzd discourse relations arise bec,~use the meanings of exl~ressions in utt(.~ra.n('(-,s, the things 1)eing ta.lked about, stand in some relation ill tlle domain of discourse. CAUSE, ['or exa.mple, is a.n iuforma.tiona.l rela.tion because it is a rela.tion between things tha.t a,r~, being ta.lked a.l)oul., tim fa('l, of on(~ situa,tion or