Why do discourses need to be coherent and what are the different kinds of coherence?

Discourses are not arbitrary collections of sentences; they must be coherent. Among the factors that make a discourse coherent are coherence relations between the sentences, entity-based coherence, and topical coherence. Various sets of coherence relations and rhetorical relations have been proposed. The relations in Rhetorical Structure Theory (RST) hold between spans of text and are structured into a tree. Because of this, shift-reduce and other parsing algorithms are generally used to assign these structures. The Penn Discourse Treebank (PDTB) labels only relations between pairs of spans, and the labels are generally assigned by sequence models. Many different genres have different types of global coherence. Persuasive essays have claims and premises that are extracted in the field of argument mining, scientific articles have structure related to aims, methods, results, and comparisons.

What is entity-based coherence?

Entity-based coherence captures the intuition that discourses are about an entity, and continue mentioning the entity from sentence to sentence. Centering Theory is a family of models describing how salience is modeled for discourse entities, and hence how coherence is achieved by virtue of keeping the same discourse entities salient over the discourse. The entity grid model gives a more bottom-up way to compute which entity realization transitions lead to coherence.