Discourse

Martin HasselKTH NADA

Royal Institute of Technology100 44 Stockholm+46-8-790 66 34

xmartin@nada.kth.se

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What is a discourse?

• The linguistic term for a contextually related group of sentences or utterances

• Basic discourse types• Monologue• Dialogue• HCI turn taking / ”dialogue”

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Cohesion and Coherence

• Cohesion • The bond that ties sentences to one another on a

textual level

• Coherence• The application of cohesion in order to form a

discourse

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Reference Phenomena 1

• Indefinite noun phrases• an apple, some lazy people

• Definite noun phrases• the fastest computer

• Demonstratives• this, that

• One-anaphora

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Reference Phenomena 2

• Inferrables• car engine, door

• Discontinous sets• they, them

• Generics• they

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Referential Constraints

• Agreement• Number• Person and case• Gender

• Syntactic constraints• Selectional restrictions

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Coreferential Expressions

• Coreference• Expressions denoting the same discourse entity corefer

• Anaphors• Refer backwards in the discourse• The referent is called the antecedent

• Cataphors• Refer forwards in the discourse

Although he loved fishing, Paul went skating with Mary.

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Pronouns 1

• A pronoun that emphasizes a person, place, or idea is demonstrative• these, that, this, those

• A pronoun that forms a question in the sentence is interrogative• whom, who, which, what, whose

• Subordinate clauses are introduced by relative pronouns• that, which, whose, whom, who

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Pronouns 2

• Possessive pronouns are used to show ownership over something else.• whose, your, its, their

• A pronoun is personal if it refers to the person speaking.  It is also personal if it refers to the person being spoken to or the person being spoken about.• all, another, many, someone, other, neither, anybody

• A person, place, or thing that is not specifically named is refered to by a indefinite pronoun.• I, my, our, yours, you, its, they, their, them

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Pronouns 3

• Seldom refer more than two sentences back• Requires a salient referent as antecedent

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Antecedent Indicators

• Recency• Grammatical role• Parallellism• Repeated mention• Verb semantics

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Text Coherence

• Coherence relations• Result• Explanation• Parallel• Elaboration• Occasion

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Coherence Relations 1

• When S0 is a first sentence and S1 is a second sentence in the same discourse:

• Result• S0 causes or can cause S1

• Explanation• S1 is (or can be) the cause of what is stated in S0

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Coherence Relations 2

• Parallel• Rabbits eat carrots. Cats eat mice.

• Elaboration• Peter bought some food. He bought two bananas and

a dietary fruit drink.

• Occasion• Peter bought some snacks. He ate in front of the TV.

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Discourse Structure

• John went to the bank to deposit his paycheck (S1)• He then took a train to Bill’s car dealership (S2)• He needed to buy a car (S3)• The company he works for now isn’t near any

public tranportation (S4)• He also wanted to talk to Bill about their softball

league (S5)

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A Discourse Tree

S2 (S2 (ee2)2)

Explanation (Explanation (ee2)2)

Occasion (Occasion (ee1;1;ee2)2)

S1 (S1 (ee1)1)

Parallel (Parallel (ee3;3;ee5)5)

S3 (S3 (ee3)3)

S5 (S5 (ee5)5)Explanation (Explanation (ee3)3)

S4 (S4 (ee4)4)

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Inference 1

Rule: If it rains the ground gets wet

Observation: It rains

Conclusion: The ground gets wet

Deduction: rule + observation → conclusion (modus ponens)

Induktion: observation +conclusion → rule (modus tollens)

Abduktion: rule + conclusion - (?!) → obeservation

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Inference 2

• John hid Bill’s car keys. He was drunk.1. John usually does stupid things when drunk

2. Bill often drives when drunk

• Bill was drunk. John hid his car keys.1. Bill tends to ”borrow” cars when drunk

2. Bill often drives his car when drunk

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Background Knowledge

• The problem of encoding inference is usually said to AI-complete

• AI-completeness indicates that the problem requires all of the knowledge – and utilities to utilize it – that humas possess

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Different Levels

• Syntax• Rules for constructing grammatical sentences

• Semantics• Rules for assigning meaning to statements

• Pragmatics• Rules (of thumb) for applying contextual constraints on

the semantics of a statement

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Pragmatics

• The study of meaning contained by utterences in situations (Leech, 1983)

• Relates the content of a clause (semantics) with the content of an utterance of that clause (pragmatics)

• Pragmatic rules often rules of thumb• Dialogues – Cooperative Principles

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Grice’ Cooperative Principle

• Grice' Cooperative Principles (Grice 1975) are a set of conversational principles that have been developed to facilitate conversation.

• These principles are usually, or should be, followed in order to effectivly convey the meaning of an utterance.

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• Quantity– Don’t say more that necessary

• Quality– Do not say anything you do not believe in or have proof

of• Relevance

– A response should be an answer to the question• Form

– Be clear– Avoid ambiguity– Be consice– Be methodical

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Discourse, what for?

• Information Retrieval• Summarization• Pronoun Resolution

…• Natural Language Generation