introduction to semantics - columbia · pdf filemorphology syntax semantics ... • tau,...
TRANSCRIPT
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IntroductiontoSemantics
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Announcements• PartofnextTuesday’sclasswillfocusonques6onsaboutthemidterm.Pleasesendtopicsthatyouwouldliketohavere-explained.Onlythosetopicsthatstudentsrequestwillbecovered.
• Reading:C14J&M,V2:probabilis6cparsingotherthandependencyparsing.
• ReadC.3ofcourseworkspostfordependencyparsing.• C.14J&M,V3:anotherreadingondependencyparsing
• HstudentswhohaveaconflictwithanotherclassforthemidtermreplytoPiazzapostoremailElsbethatect2150@columbia.edu• Op6onswillbelaterThursdayorMondayevening
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SIRI: One shot Question and answer What’s required?
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WatsonandjeopardyBangBang"his"Silverhammercamedownuponherhead"
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Question-Answering/Dialog• “Match”
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Wheredoestheinformationfortheanswerscomefrom?
• Databases• hWp://newyork.citysearch.com
• Theweb
• Dic6onaries
• Inference
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OneViewofNLArchitecture
Syntax Semantics Morphology
Knowledge Representation/
Meaning Representation
Application output
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OneViewofNLArchitecture
Syntax
Semantics
Morphology
Knowledge Representation/
Meaning Representation
Application output
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AnotherView:UnderstandinginWatson• Massiveparallelism
• Exploitmassiveparallelismintheconsidera6onofmul6pleinterpreta6onsandhypotheses.
• Manyexperts• Facilitatetheintegra6on,applica6on,andcontextualevalua6onofawiderangeoflooselycoupledprobabilis6cques6onandcontentanaly6cs.
• Pervasiveconfidenceintegra6on• Nocomponentcommitstoananswer;allcomponentsproducefeaturesandassociatedconfidences,scoringdifferentques6onandcontentinterpreta6ons.
• Integrateshallowanddeepknowledge• Balancetheuseofstrictseman6csandshallowseman6cs,leveragingmanylooselyformedontologies.
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WhatisSemantics?• MeaningRepresenta6on
• Seman6cParsingorSeman6cInterpreta6on
• Wordmeaningdisambigua6on
• Entailmentandinference
• Informa6onextrac6on
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MeaningRepresentation• Torepresentques6ons
• Torepresentknowledgedrawnfromtext
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WhatCanServeasaMeaningRepresentation?
• Anythingthatallowsusto• Answerques6ons(WhatisthebestFrenchrestaurantintheEastVillage?)
• Determinetruth(IsColumbiaUnivinNYC?)• Drawinferences(IfOneDineRestaurantisonthetopflooroftheFreedomTowerandtheFreedomToweristhetallestbuildinginNewYorkCity,thenOneDineisthetopmostrestaurantinNewYorkCity.)
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Whatkindsofmeaningdowewanttocapture?
• Categories/en66es• Tau,Jane,Asiancuisine,vegetarian
• Events• takingataxi,destruc6onofpowergridinPuertoRico
• Time• Oct30,nextweek,in2months
• Aspect• Kathyknowshowtorun.Kathyisrunning.Kathyrantotherestaurantin5min.
• Beliefs,DesiresandInten6ons(BDI)
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MeaningRepresentations
• Allrepresent‘linguis6cmeaning’of Ihaveacar
andstateofaffairsinsomeworld• Allconsistofstructures,composedofsymbolsrepresen6ngobjectsandrela6onsamongthem• FOPC:
)}(),(),()({, yCarxyHadThingxSHaverxHavingyx ∧∧∧∃
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AStandardRepresentation:Predicate-ArgumentStructure
• Representsconceptsandrela6onshipsamongthem• Nounsasconceptsorarguments(red(ball))• Adjec6ves,adverbs,verbsaspredicates(red(ball))
• Subcategoriza6on(or,argument)framesspecifynumber,posi6on,andsyntac6ccategoryofarguments• NPlikesNP• NPlikesInf-VP• NPlikesNPInf-VP
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Semantic(Thematic)Roles• Subcatframeslinkargumentsinsurfacestructurewiththeirseman6croles• Agent:GeorgehitBill.BillwashitbyGeorge.• Pa6ent:GeorgehitBill.BillwashitbyGeorge.
• Theclaimofatheoryofseman6crolesisthattheseargumentsofpredicatescanbeusefullyclassifiedintoasmallsetofseman6callycontenlulclasses• Andthattheseclassesareusefulforexplaininglotsofthings
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Commonsemanticroles• Agent:ini6atorordoerintheevent• Pa(ent:affecteden6tyintheevent;undergoestheac6on• Suekilledtherat.
• Theme:objectintheeventundergoingachangeofstateorloca6on,orofwhichloca6onispredicated• Theicemelted
• Experiencer:feelsorperceivetheevent• Billlikespizza.
• S(mulus:thethingthatisfeltorperceived
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Commonsemanticroles• Goal:
• BillrantoCopleySquare.• Recipient(mayormaynotbedis6nguishedfromGoal):
• BillgavethebooktoMary.
• Benefac(ve(maybegroupedwithRecipient):• BillcookeddinnerforMary.
• Source:• Billtookapencilfromthepile.
• Instrument:• Billatetheburritowithaplas6cspork.
• Loca(on:• BillsitsunderthetreeonWednesdays
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CommonsemanticrolesTryforyourself!1. Thesubmarinesankatroopship.2. Dorishidthemoneyintheflowerpot.3. Emmano6cedthestain.4. Wecrossedthestreet.5. Theboysclimbedthewall.6. Thechefcookedagreatmeal.7. Thecomputerpinpointedtheerror.8. AmadbulldamagedthefenceonJack’sfarm.9. ThecompanywrotemealeWer.10. Jackopenedthelockwithapaperclip.
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Linkingofthematicrolestosyntacticpositions• Johnopenedthedoor• AGENTTHEME• ThedoorwasopenedbyJohn• THEMEAGENT• Thedooropened• THEME• Johnopenedthedoorwiththekey• AGENTTHEMEINSTRUMENT
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DeeperSemantics• FromtheWSJ…
• Hemeltedherreservewithahusky-voicedpaeantohereyes.
• Ifwelabelthecons6tuentsHeandherreserveastheMelterandMelted,thenthoselabelsloseanymeaningtheymighthavehad.
• IfwemakethemAgentandThemethenwecandomoreinference.
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SelectionalRestrictions• Selec6onalRestric6ons:constraintsonthetypesofargumentsverbstakeGeorgeassassinatedthesenator.*Thespiderassassinatedthefly.assassinate:inten5onal(poli5cal?)killing
• Theastronautmarriedthestar.
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Problems• Whatexactlyisarole?• What’stherightsetofroles?• Aresuchrolesuniversals?• Aretheserolesatomic?
• I.e.Agents• Animate,Voli6onal,Directcausers,etc
• Canweautoma6callylabelsyntac6ccons6tuentswiththema6croles?
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FirstOrderPredicateCalculus• Notidealasameaningrepresenta6onanddoesn'tdoeverythingwewant--butbeWerthanmany…• Supportsthedetermina6onoftruth• Supportscomposi6onalityofmeaning• Supportsques6on-answering(viavariables)• Supportsinference
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AbstractMeaningRepresentation(AMR)• Aimingtocaptureadeeplevelofseman6cs
• Originalgoal:asinterlinguaformachinetransla6on
• Alsonowsee:AMRparsers,genera6onfromAMR,summariza6onusingAMR
Knight, AMR Release 1.0, 2014
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TemporalRepresentations• Howdowerepresent6meandtemporalrela6onshipsbetweenevents?ItseemsonlyyesterdaythatMarthaStewartwasinprisonbutnowshehasapopularTVshow.Thereisnojus6ce.
• Wheredowegettemporalinforma6on?• Verbtense• Temporalexpressions• Sequenceofpresenta6on
• Linearrepresenta6ons:Reichenbach‘47
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RepresentingTime
Example from Russell and Norvig
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RepresentingTime
Example from Jurafsky and Martin
Utterance time (U): when the utterance occurs Reference time (R): the temporal point-of-view of the utterance Event time (E): when events described in the utterance occur
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VerbsandEventTypes:Aspect• Sta6ves:statesorproper6esofobjectsatapar6cularpointin6me
Iamhungry.• Ac6vi6es:eventswithnoclearendpoint
Iamea-ng.• Accomplishments:eventswithdura6onsandendpointsthatresultinsomechangeofstate
Iatedinner.• Achievements:eventsthatchangestatebuthavenopar6culardura6on–theyoccurinaninstant
Igotthebill.
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Beliefs,DesiresandIntentions• Veryhardtorepresentinternalspeakerstateslikebelieving,knowing,wan6ng,assuming,imagining• NotwellmodeledbyasimpleDBlookupapproachso..
• Truthintheworldvs.truthinsomepossibleworldGeorgeimaginedthathecoulddance.Georgebelievedthathecoulddance.
• AugmentFOPCwithspecialmodaloperatorsthattakelogicalformulaeasarguments,e.g.believe,know
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Believes(George,dance(George))Knows(Bill,Believes(George,dance(George)))
• Mutualbelief:IbelieveyoubelieveIbelieve….• Prac6calimportance:modelingbeliefindialogue
• Clark’sgrounding
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TakeAway• Seman6csmeansmanythings
• Meaningrepresenta6ons• Seman6cparsingorseman6crolelabeling• Determiningwordmeaning• Useofseman6cinforma6onforanendapplca6on• Inference• Temporalrela6ons:tense,aspect• Informa6onextrac6on• Belief,desireandintent
• Workinseman6cscanfallintoanyoftheseareas
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LexicalSemantics• Groundingforrepresenta6onofwordmeaning• Usedforworddisambigua6on• Seman6cinterpreta6on
• Pre-cursortorepresenta6onsusedindistribu6onalseman6cs,vectorrepresenta6onsandneuralnets
• Topics• Homonymy,Polysemy,Synonymy• Onlineresources:WordNet
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Preliminaries• What’saword?
• Defini6onswe’veusedovertheclass:Types,tokens,stems,roots,inflectedforms,etc...
• Lexeme:Anentryinalexiconconsis6ngofapairingofaformwithasinglemeaningrepresenta6on
• Lexicon:Acollec6onoflexemes
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Relationshipsbetweenwordmeanings• Homonymy• Polysemy• Synonymy• Antonymy• Hypernomy• Hyponomy• Meronomy
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Homonymy• Homonymy:
• Lexemesthatshareaform• Phonological,orthographicorboth
• Buthaveunrelated,dis6nctmeanings• Clearexamples:
• Bat(woodens6ck-likething)vs• Bat(flyingscarymammalthing)• Orbank(financialins6tu6on)versusbank(riverside)
• Canbehomophones,homographs,orboth:• Homophones:
• Writeandright• Pieceandpeace
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HomonymycausesproblemsforNLPapplications• Text-to-Speech
• Sameorthographicformbutdifferentphonologicalform• bassvsbass
• Informa6onretrieval• Differentmeaningssameorthographicform
• QUERY:batcare• MachineTransla6on• Speechrecogni6on
• Why?
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Polysemy• ThebankisconstructedfromredbrickIwithdrewthemoneyfromthebank
• Arethosethesamesense?• OrconsiderthefollowingWSJexample
• Whilesomebanksfurnishspermonlytomarriedwomen,othersarelessrestric6ve
• Whichsenseofbankisthis?• Isitdis6nctfrom(homonymouswith)theriverbanksense?
• Howaboutthesavingsbanksense?
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Polysemy• Asinglelexemewithmul6plerelatedmeanings(bankthebuilding,bankthefinancialins6tu6on)
• Mostnon-rarewordshavemul6plemeanings• Thenumberofmeaningsisrelatedtoitsfrequency
• Verbstendmoretopolysemy• Dis6nguishingpolysemyfromhomonymyisn’talwayseasy(ornecessary)
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MetaphorandMetonymy• Specifictypesofpolysemy• Metaphor:
• GermanywillpullSloveniaoutofitseconomicslump.
• Ispent2hoursonthathomework.• Metonymy
• TheWhiteHouseannouncedyesterday.• Thischaptertalksaboutpart-of-speechtagging
• Bank(building)andbank(financialins6tu6on)
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Howdoweknowwhenawordhasmorethanonesense?• ATISexamples
• Whichflightsservebreakfast?• DoesAmericaWestservePhiladelphia?
• The“zeugma”test:
• ?DoesUnitedservebreakfastandSanJose?
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Synonyms• Wordthathavethesamemeaninginsomeorallcontexts.• filbert/hazelnut• couch/sofa• big/large• automobile/car• vomit/throwup• Water/H20
• Twolexemesaresynonymsiftheycanbesuccessfullysubs6tutedforeachotherinallsitua6ons• Ifsotheyhavethesameproposi(onalmeaning
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Synonyms• Buttherearefew(orno)examplesofperfectsynonymy.• Whyshouldthatbe?• Evenifmanyaspectsofmeaningareiden6cal• S6llmaynotpreservetheacceptabilitybasedonno6onsofpoliteness,slang,register,genre,etc.
• Example:• WaterandH20
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Somemoreterminology• Lemmasandwordforms
• Alexemeisanabstractpairingofmeaningandform• Alemmaorcita(onformisthegramma6calformthatisusedtorepresentalexeme.
• Carpetisthelemmaforcarpets• Dormiristhelemmaforduermes.
• Specificsurfaceformscarpets,sung,duermesarecalledwordforms
• Thelemmabankhastwosenses:• Instead,abankcanholdtheinvestmentsinacustodialaccountintheclient’sname
• Butasagricultureburgeonsontheeastbank,theriverwillshrinkevenmore.
• Asenseisadiscreterepresenta6onofoneaspectofthemeaningofaword
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Synonymyisarelationbetweensensesratherthanwords
• Considerthewordsbigandlarge• Aretheysynonyms?
• Howbigisthatplane?• WouldIbeflyingonalargeorsmallplane?
• Howabouthere:• MissNelson,forinstance,becameakindofbigsistertoBenjamin.
• ?MissNelson,forinstance,becameakindoflargesistertoBenjamin.
• Why?• bighasasensethatmeansbeingolder,orgrownup• largelacksthissense
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Antonyms• Sensesthatareoppositeswithrespecttoonefeatureoftheirmeaning
• Otherwise,theyareverysimilar!• dark/light• short/long• hot/cold• up/down• in/out
• Moreformally:antonymscan• defineabinaryopposi6onoratoppositeendsofascale(long/short,fast/slow)
• Bereversives:rise/fall,up/down
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Hyponymy• Onesenseisahyponymofanotherifthefirstsenseismorespecific,deno6ngasubclassoftheother• carisahyponymofvehicle• dogisahyponymofanimal• mangoisahyponymoffruit
• Conversely• vehicleisahypernym/superordinateofcar• animalisahypernymofdog• fruitisahypernymofmango
superordinate vehicle fruit furniture mammal
hyponym car mango chair dog
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Hypernymymoreformally• Extensional:
• Theclassdenotedbythesuperordinate• extensionallyincludestheclassdenotedbythehyponym
• Entailment:• AsenseAisahyponymofsenseBifbeinganAentailsbeingaB
• Hyponymyisusuallytransi6ve• (AhypoBandBhypoCentailsAhypoC)
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• Whywouldhypernyms/hyponymsbeimportanttoconstruc6ngameaningrepresenta6on?
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II.WordNet• Ahierarchicallyorganizedlexicaldatabase• On-linethesaurus+aspectsofadic6onary
• Versionsforotherlanguagesareunderdevelopment
Category Unique Forms
Noun 117,097
Verb 11,488
Adjective 22,141
Adverb 4,601
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WordNet
• Whereitis:• hWps://wordnet.princeton.edu/
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FormatofWordnetEntries
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WordNetNounRelations
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WordNetVerbRelations
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WordNetHierarchies
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Howis“sense”deYinedinWordNet?• Thesetofnear-synonymsforaWordNetsenseiscalledasynset(synonymset);it’stheirversionofasenseoraconcept
• Example:chumpasanountomean• ‘apersonwhoisgullibleandeasytotakeadvantageof’
• Eachofthesesensessharethissamegloss• ThusforWordNet,themeaningofthissenseofchumpisthislist.
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NextThursday:Wordsensedisambigua6onTuesday:Seman6cparsingandmidterm