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Semantics

Semantics. Going beyond syntax. Semantics. Relationship between surface form and meaning What is meaning? Lexical semantics Syntax and semantics. What is meaning?. Reference to “worlds” Objects, relationships, events, characteristics Meaning as truth Understanding

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Semantics

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  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

  7. 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

  9. 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

  10. 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

  11. 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)

  12. Types of representation 6. Conceptual dependency (Schank) John punched Mary

  13. Types of representation 7. Semantic formulae (Wilks) ((THIS((PLANT STUFF)SOUR)) ((((((THRU PART)OBJE) (NOTUSE *ANI))GOAL) ((MAN USE) (OBJE THING) ))) door

  14. Uses for semantic representations • 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 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

  15. 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

  16. 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.

  17. 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)

  18. 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

  19. WordNet • Began as a psycholinguistic “theory” of how the brain organizes its vocabulary (Miller) • Organizes vocabulary into “synsets”, hierarchically arranged together with other relations (hyp[er|o]nymy, isa, member, antonyms, entailments) • Turns out to be very useful for many applications • Has been replicated for many languages (sometimes just translated!)

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