Discourse Representation Theory

Presented by Mark Chang

Discourse Phenomena

● Discourse is a sequences of sentences.● When we look at discourse, interesting

challenges arise.● One of these challenges is interpreting

pronouns, such as he, she and it.

Anaphoric Pronouns

● A woman walks. She smokes.

Need to Expand the scope of the existential quantifier.

Donkey Sentences

● Every farmer that owns a donkey beats it.

Expand the quantifier scope

It seems that the sentence is correct,However...

Donkey Sentences

● Every farmer that owns a donkey beats it.

farmer

donkey

pigown

own

pigfarmerfarmer

beat

Donkey Sentences

● Every farmer that owns a donkey beats it.

farmerpigown Pig farmer

beat

donkeyown

donkeyfarmerbeat

False

False

True

True

TrueFalse

Discourse Representation Theory (DRT)

● A formalism proposed in the early 1980s by Hans Kamp● The invention of DRT led to a shift from a static to a

dynamic view on natural language semantics.● Instead of working with first-order formula syntax, works with

explicit semantic representations.● Discourse Representation Structure (DRS) describes the

objects mentioned in a discourse and their properties.

A First Example of DRS

● A woman walks. She smokes.

discourse referent x, y, in the top part of the box.

conditions upon these discourse referents in the lower part of the box.

Syntax of DRSs and DRS-Conditions

● Structure of DRS

● DRS-conditions

DRS-conditions

discourse referents

Syntax of DRSs and DRS-Conditions

● DRS-conditions

Subordination

● B subordinates B' if and only if

Accessibility

● B is accessible from B' if and only if– B subordinates from B'

– B and B' denote the same DRS

● Accessibility is the criterion for the antecedent of anaphoric pronoun.

● Actually, Many factors play a role in pronoun resolution, such as prosodic and syntactic information and common-sense knowledge

Discourse Structure and Accessibility

● Mary ordered a milk shake,

John tasted it.

The discourse referent y is accessible for discourse referent v.

An anaphoric link between it and milk shake is allowed.

Discourse Structure and Accessibility

● Mary did not ordered a milk shake.

John tasted it.

When we introduced negation, an anaphoric link is blocked.

Hence, y is not accessible for v

Donkey Sentences

● Every farmer that owns a donkey beats it.

Following the definition of accessibility, the discourse referent y introduced by a donkey is available as antecedent.

A link is established by the DRS-condition z = y.

Reference

● Computational Semantics– http://www.coli.uni-saarland.de/projects/milca/courses/comsem/html/node205.html