Towards a Discourse Level Coherence Structure

In this paper we argue that coherence relations between discourse units are ultimately based on mentioned discourse entities embedded in the units participating in the relation. Coherence relations as discussed in most literature ((Mann and Thompson, 1988), (Hobbs, 1985), (Grosz and Sidner, 1986) inter alia) are defined between text segments, where a text segment could range from a single utterance to the whole discourse. We show that these coherence relations are formed either directly or indirectly between embedded discourse entities. Other semantic entities might be derived via inference/s based on the mentioned entities and the complexity of these inferences determines some of the types of relations defined in literature. Hence, the coherence relations as defined by (Mann and Thompson, 1988), (Hobbs, 1985) inter alia, existing between text units is essentially an abstraction of these fundamental relations formed between embedded entities. We argue that any representation of discourse coherence structure should entail representation of information down to the resolution level of these embedded entities in order for such structures to be useful for automated language processing tasks. We also show that the commonly accepted tree structure ((Hobbs, 1985),(Marcu, 1996) inter alia) is not sufficient to represent discourse relations to such a resolution level, and propose a semi constrained directed graph as the alternative.