Semantics

Semantics: 

Semantics Going beyond syntax

Semantics: 

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

What is meaning?: 

What is meaning? Reference to 'worlds' Objects, relationships, events, characteristics Meaning as truth Understanding Inference, implication Modelling beliefs Meaning as action Understanding activates procedures

Lexical semantics: 

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

Lexical relations: 

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

Semantic features: 

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

Types of representation: 

Types of representation The man shot an elephant with his gun shot subj obj adv man elephant gun det det mod the an his 1. Syntactic relations

Types of representation: 

Types of representation The man shot an elephant with his gun shot dsubj dobj instr man elephant gun qtf qtf poss the an his 2. Deep syntax An elephant was shot by the man with his gun

Types of representation: 

Types of representation The man shot an elephant with his gun shot agent patient instr man elephant gun qtf qtf poss the an his 3. Semantic roles, deep cases An elephant was shot by the man with his gun The man used his gun to shoot an elephant

Types of representation: 

Types of representation The man shot an elephant with his gun shooting shooter shot- instr thing man elephant gun qtf qtf poss the  man 4. Event representation, semantic network An elephant was shot by the man with his gun The man used his gun to shoot an elephant

Types of representation: 

Types of representation The man owned the gun which he used to shoot an elephant 5. Predicate calculus An elephant was shot by the man with his gun The man used his gun to shoot an elephant event(e) andamp; time(e,past) andamp; pred(e,shoot) andamp; man(A) andamp; the(A) andamp; (B) andamp; dog(B) andamp; shoot(A,B) andamp; (C) andamp; gun(C) andamp; own(A,C) andamp; use(A,C,e) The man shot an elephant with his gun The man used the gun which he owned to shoot an elephant

Types of representation: 

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

Types of representation: 

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

Uses for semantic representations: 

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

Uses for semantic representations: 

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

Linguistic issues: 

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.

Linguistic issues: 

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)

Lexical semantics: 

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

WordNet: 

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!)