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Semantics

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  • 1. 1/27SemanticsGoing beyond syntax
  • 2. 2/27Semantics• Relationship between surface form andmeaning• What is meaning?• Lexical semantics• Syntax and semantics
  • 3. 3/27What is meaning?• Reference to “worlds”– Objects, relationships, events, characteristics– Meaning as truth• Understanding– Inference, implication– Modelling beliefs• Meaning as action– Understanding activates procedures
  • 4. 4/27Lexical semantics• Meanings of individual words– Sense and Reference– What do we understand by the word lio n ?– Is a toy lion a lion? Is a toy gun a gun? Is a fakegun a gun?• Grammatical meaning– What do we understand by the lio n, lio ns, thelio ns, … as inThe lio n is a dang e ro us anim alThe lio n was abo ut to attack
  • 5. 5/27Lexical relations• Lexical meanings can be defined interms of other words– Synonyms, antonyms, broader/narrowerterms– synsets– Part-whole relationships (often reflect real-world relationships)– Linguistic usage (style, register) also afactor
  • 6. 6/27Semantic features• Meanings can be defined (to a certainextent) in terms of distinctive features– e.g. m an = adult, male, human• Meanings can be defined (to a certainextent) in terms of distinctive features
  • 7. 7/27Types of representationThe man shot an elephant with his gunshotsubj obj advman elephant gundet det modthe an his1. Syntactic relations
  • 8. 8/27Types of representationThe man shot an elephant with his gunshotdsubj dobj instrman elephant gunqtf qtf possthe an his2. Deep syntaxAn elephant was shot by the man with his gun
  • 9. 9/27Types of representationThe man shot an elephant with his gunshotagent patient instrman elephant gunqtf qtf possthe an his3. Semantic roles, deep casesAn elephant was shot by the man with his gunThe man used his gun to shoot an elephant
  • 10. 10/27Types of representationThe man shot an elephant with his gunshootingshooter shot- instrthingman elephant gunqtf qtf possthe ∃ man4. Event representation, semantic networkAn elephant was shot by the man with his gunThe man used his gun to shoot an elephant
  • 11. 11/27Types of representationThe man owned the gun which he used to shoot an elephant5. Predicate calculusAn elephant was shot by the man with his gunThe man used his gun to shoot an elephantevent(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)The man shot an elephant with his gunThe man used the gun which he owned to shoot an elephant
  • 12. 12/27Types of representation6. Conceptual dependency (Schank)John punched Mary
  • 13. 13/27Types of representation7. Semantic formulae (Wilks)door((THIS((PLANT STUFF)SOUR))((((((THRU PART)OBJE) (NOTUSE *ANI))GOAL)((MAN USE) (OBJE THING))))
  • 14. 14/27Uses for semanticrepresentations• As a linguistic artefact (because it’s there)• To capture the text ⇔ meaning relationship• Identifying paraphrases, equivalences (e.g.summarizing a text, searching a text forinformation)• Understanding and making inferences (e.g.so as to understand a sequence of events)• Interpreting questions (so as to find theanswer), commands (so as to carry themout), statements (so as to update data)• Translating
  • 15. 15/27Uses for semanticrepresentations• Different levels of understanding/meaning• Textual meaning may be little more thandisambiguating– Attachment ambiguities– Word-senses– Anaphora (pronoun reference, coreference)• Conceptual meaning may be much deeper• Somewhere in between – a good example isWilks’ preference semantics: especially goodfor metaphor
  • 16. 16/27Linguistic issues• Words and Concepts– Objects, properties, actions ≈ n, adj, v– Language allows us to be vague (e.g. to y g un)• Semantic primitives – what are they?• Meaning equivalence – when do two thingsmean the same?• Grammatical meaning– Tense vs. time– Topic and focus– Quantifiers, plurals, etc.
  • 17. 17/27Linguistic issues• There are many other similarly trickylinguistic phenomena– Modality (could, should, would, must, may)– Aspect (completed, ongoing, resulting)– Determination (the, a, some, all, none)– Fuzzy sets (often, some, many, usually)
  • 18. 18/27Lexical 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, holdsmuch of the “knowledge” used by cleversystems
  • 19. 19/27WordNet• Began as a psycholinguistic “theory” of howthe brain organizes its vocabulary (Miller)• Organizes vocabulary into “synsets”,hierarchically arranged together with otherrelations (hyp[er|o]nymy, isa, member,antonyms, entailments)• Turns out to be very useful for manyapplications• Has been replicated for many languages(sometimes just translated!)