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3.6. DISCOURSE AND PRAGMATIC ANALYSIS

Up to now, we have dealt mainly with the analysis of single statements, written sentences, or utterances -- even if, in the previous sections, we have seen how advanced knowledge representation systems like Conceptual Graphs or NKRL allow one to perform a sort of "clustering" of the "meaning" of several statements making use of second order mechanisms like PROPOSITIONs and SITUATIONs in Conceptual Graphs, or binding structures in NKRL. There exists, however, a specific branch of Linguistics that deals, in a systematic way, with the problem of studying the logical and semantic connections that normally exist, in a text or a dialog, among the different statements: this is the "discourse analysis." Discourse analysis deals, in general, with the two following problems: (1) determining the nature of the information that, in a sequence of statements, goes beyond the simple addition of the information conveyed by the single sentences or utterances; and (2) determining the influence of the context in which a statement is used on the meaning of the individual statement, or part of it.

Especially when the generic "statements" become, more precisely, spoken utterances, "pragmatic analysis" -- i.e., how to use the global, final analysis of a linguistic sequence to perform a particular task -- interacts strongly with discourse analysis. To determine, in fact, the pragmatic properties of a sequence of utterances -- i.e., what kinds of "speech acts" the speakers are performing (are they asking a question, answering a previous question, making a proposal, etc.) -- it is often necessary to have a global representation of their meaning. Conversely, in order to identify this global meaning it is often necessary to determine exactly the pragmatic role the sequence of utterances is meant to play.

The difficulty in obtaining, in these fields, results of a general import is well evident, and the availability of concrete tools is still scarce. With respect to the discourse analysis of written texts, early work was accomplished in the 1970s in the context of the so-called "text grammars," intended to define the conditions that must be satisfied for a text to be coherent -- for example, the "condition of introduction" requires that an entity must be introduced, e.g., by an existential quantifier, before it can used as a discourse referent, e.g., in a definite noun phrase. Other work that dates back to the 1970s is the work originated by the introduction of the notion of "script" by Schank and Abelson (see subsection 2.3), and the introduction, by Schank and disciples, of tools like MOPs ("Memory Organization Packets") or TAUs ("Thematic Affect Units"). Unlike scripts, which only consider the actions performed by some characters within a given setting, MOPs include goals and intentions; TAUs have, among other things, a strong "affective" component, i.e., they can account for the different feelings of the characters.

More recent work in the area of discourse analysis of written texts includes that of Hahn, Hobbs, and Schubert. In the TOPIC system, Hahn deals with the detection of the "thematic progression patterns" that represent the global structure of the input texts: this is mainly performed by analyzing the semantic links between the concepts represented by nominal expressions in the texts. TOPIC is then able to detect various "coherence phenomena" that bring about some congruent aspects of the different "chunks of knowledge" isolated inside an NL text, like the "constant theme," i.e., the possibility to show that a variety of facts are, in reality, related to the same topic. Hobbs' work on "interpretation as abduction" is based on the theoretical assumption that the coherence of discourse follows from semantic relationships between the information conveyed by the successive statements -- abductive reasoning is based on inferences of the type: "if A implies B and B has been observed, hypothesize A," and consists in computationally intensive inference techniques in a backward chaining style. After having executed the parse of a given statement into quasilogical form (see subsection 3.5.1), Hobbs' TACITUS system tries to prove, allowing a minimal set of assumptions to be made, that this logical form stems from the preceding statements and from the adherence to some form of general explanatory schema for the global text. The optimum (most complete) set of assumptions bringing out the proof can then be regarded as the best "explanation" of this text. Schubert's "Episodic Logic" (EL) is based on a Montague style of representation coupling syntactic forms and logical forms, while incorporating from situation semantics the idea that sentences describe situations (events, states, narrative episodes, eventualities, etc.). From the point of view of the text's coherence, EL makes use, among other things, of "episodic variables," which are used to make implicit temporal and causal relationships between situations fully explicit.

With respect to the discourse analysis of dialogs, we can mention here a theoretical work, Kamp's "Discourse Representation Theory" (DRT), a complex sort of "intermediate conceptual representation" devoted to the formalization of the connectivity phenomena, that has been expressly developed for the purpose of representing and computing trans-sentential anaphora, contexts, and other forms of discourse cohesion. In spite of his very general, theoretical framework, DRT has also been used for the design of some concrete prototypes, in particular for the design of advanced experimental question-answering systems. In a recent (1993) project like VERBMOBIL, e.g., DRT is used as unifying semantic representation formalism for developing a machine translation system taking into account face-to-face spoken dialogs.


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