Semantics

1
Semantics

• Going beyond syntax

2
Semantics

• Relationship between surface form and meaning
• What is meaning?
• Lexical semantics
• Syntax and semantics

3
What is meaning?

• Reference to worlds
• Objects, relationships, events, characteristics
• Meaning as truth
• Understanding
• Inference, implication
• Modelling beliefs
• Meaning as action
• Understanding activates procedures

4
Lexical semantics

• Meanings of individual words
• Sense and Reference
• What do we understand by the word lion ?
• Is a toy lion a lion? Is a toy gun a gun? Is a
  fake gun a gun?
• Grammatical meaning
• What do we understand by the lion, lions, the
  lions, as in
• The lion is a dangerous animal
• The lion was about to attack

5
Lexical relations

• Lexical meanings can be defined in terms of other
  words
• Synonyms, antonyms, broader/narrower terms
• synsets
• Part-whole relationships (often reflect
  real-world relationships)
• Linguistic usage (style, register) also a factor

6
Semantic features

• Meanings can be defined (to a certain extent) in
  terms of distinctive features
• e.g. man adult, male, human
• Meanings can be defined (to a certain extent) in
  terms of distinctive features

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Types of representation
1. Syntactic relations
The man shot an elephant with his gun
shot subj obj adv man elephant
gun det det mod the an
his
8
Types of representation
2. Deep syntax
The man shot an elephant with his gun
An elephant was shot by the man with his gun
shot dsubj dobj instr man
elephant gun qtf qtf
poss the an his
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Types of representation
3. Semantic roles, deep cases
The man shot an elephant with his gun
An elephant was shot by the man with his gun
shot agent patient instr man elephant
gun qtf qtf poss the
an his
The man used his gun to shoot an elephant
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Types of representation
4. Event representation, semantic network
The man shot an elephant with his gun
An elephant was shot by the man with his gun
shooting shooter shot- instr thing man
elephant gun qtf qtf
poss the ? man
The man used his gun to shoot an elephant
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Types of representation
5. Predicate calculus
The man shot an elephant with his gun
An elephant was shot by the man with his gun
The man used his gun to shoot an elephant
The man owned the gun which he used to shoot an
elephant
The man used the gun which he owned to shoot an
elephant
event(e) time(e,past) pred(e,shoot) man(A)
the(A) ?(B) dog(B) shoot(A,B) ?(C)
gun(C) own(A,C) use(A,C,e)
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Types of representation
6. Conceptual dependency (Schank)
John punched Mary
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Types of representation
7. Semantic formulae (Wilks)
((THIS((PLANT STUFF)SOUR)) ((((((THRU PART)OBJE)
(NOTUSE ANI))GOAL) ((MAN USE) (OBJE THING) )))
door
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Uses for semantic representations

• As a linguistic artefact (because its there)
• To capture the text ? meaning relationship
• Identifying paraphrases, equivalences (e.g.
  summarizing a text, searching a text for
  information)
• Understanding and making inferences (e.g. so as
  to understand a sequence of events)
• Interpreting questions (so as to find the
  answer), commands (so as to carry them out),
  statements (so as to update data)
• Translating

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Uses for semantic representations

• Different levels of understanding/meaning
• Textual meaning may be little more than
  disambiguating
• Attachment ambiguities
• Word-senses
• Anaphora (pronoun reference, coreference)
• Conceptual meaning may be much deeper
• Somewhere in between a good example is Wilks
  preference semantics especially good for
  metaphor

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Linguistic issues

• Words and Concepts
• Objects, properties, actions ? n, adj, v
• Language allows us to be vague (e.g. toy gun)
• Semantic primitives what are they?
• Meaning equivalence when do two things mean the
  same?
• Grammatical meaning
• Tense vs. time
• Topic and focus
• Quantifiers, plurals, etc.

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Linguistic issues

• There are many other similarly tricky linguistic
  phenomena
• Modality (could, should, would, must, may)
• Aspect (completed, ongoing, resulting)
• Determination (the, a, some, all, none)
• Fuzzy sets (often, some, many, usually)

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Lexical semantics

• Lexical relations (familiar to linguists) have an
  impact on NLP systems
• Homonymy word-sense selection homophones in
  speech-based systems
• Polysemy understanding narrow senses
• Synonymy lexical equivalence
• Ontology structure vocabulary, holds much of
  the knowledge used by clever systems

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WordNet

• Began as a psycholinguistic theory of how the
  brain organizes its vocabulary (Miller)
• Organizes vocabulary into synsets,
  hierarchically arranged together with other
  relations (hyperonymy, isa, member, antonyms,
  entailments)
• Turns out to be very useful for many applications
• Has been replicated for many languages (sometimes
  just translated!)