Purpose within discourse

Grosz and Litman [5] describe a model for discourse structure as a whole comprising three aspects: The linguistic structure, the intentional structure (structure of purposes), and the attentional state (state of attention focus).

In this model, linguistic structure corresponds roughly to the kind of structure that TextTiling attempts to discern in [6], and that Kozima [9] describes--The topic-based grouping of sentences into contiguous chunks. The intentional structure contains the purposes of the discourse which are expressed by the content of the linguistic structure. The attentional structure is about things--it records objects, properties and relations currently under discussion within the discourse.

Grosz and Litman hope to provide the foundation of an account of discourse meaning, based on this account of discourse structure.

Of direct relevance to topic segmentation, Grosz and Litman address the issue of what it is that makes a group of sentences constitute a single discourse, or several. They define two kinds of speaker within any `segment' of discourse: The Initiating Conversational Participant, and the Other Conversational Participants (ICP and OCP). Each segment is by definition initiated by the ICP, and then subsequently discussed by the OCPs (and potentially the ICP as well). This model introduces the intentional model described above: The ICP is spearheading a topic of his or her own choosing each time--flow through multiple topics does not occur directionlessly.

Grosz and Litman reinforce the assertion in [7] that there is a relationship between the utterances constituting a discourse and the discourse structure. They show that it is a two way relationship: Discourse structure is indicated and controlled by cue phrases (here also referred to as ``clue words''), but the meaning of cue phrases is determined in part by their context--that is, the discourse structure they inhabit. Again, this strongly recalls [7].

It is theorised that intentional structure is important to understanding the coherence of a discourse; specifically, whether a series of utterances comprises more than one discourse (for which we may wish to substitute the word ``topic''). The theory states that those participating in a conversation have an aim in mind: Their Discourse Purpose (DP). The DP motivates both the act of speaking at all, and the particular message conveyed by that speech.

Finally, attentional state is given by a focus space. There is a focus space for every discourse segment, containing the properties, objects and relations pertaining to that segment. The focus space does not contain the Discourse Purpose as it is not intentional--it merely describes the state of affairs in terms of objects.

A discourse is said to be coherent when all of its Conversational Participants share a Discourse Purpose, and each utterance of the discourse contributes in some way to achieving this purpose.