Logic, Information and Conversation - Semantics Archive

104 Pages·2007·0.67 MB·English

Logic,
Information
&
Conversation
c1998PaulPiwek
P(cid:0)rintedbyPrintPartnersIpskamp,TheNetherlands
CoverDesignandLayOutbyPaulPiwekinLATEX
Logic, Information and Conversation Ditproefschriftisgoedgekeurddoordepromotoren:
prof.dr.D.G.Bouwhuis
en
prof.dr.H.C.Bunt
Copromotor:
dr.ir.R.J.Beun
PROEFSCHRIFT
terverkrijgingvandegraadvandoctoraandeTechnische
UniversiteitEindhoven,opgezagvandeRectorMagnificus,
prof.dr. M.Rem,vooreencommissieaangewezendoorhet
CollegevoorPromotiesinhetopenbaarteverdedigenop
dinsdag10november1998om16.00uur
door
PaulLeoAntoinePiwek
geborenteVenlo
Tomyparents
offworkinthediscosofplacessuchasGeldern,MarmarisandL’Escala.
Brighton,September1998
Acknowledgments
First,IwouldliketothankmysupervisorsDonBouwhuis,HarryBuntandRobbert-
JanBeunforgivingmetheopportunitytowritethisdissertation.
Thanks are due to Rene´ Ahn, Tijn Borghuis, Anita Cremers, Emiel Krah-
mer,Marie¨tTheuneandKeesvanDeemterforcommentingonthecontentsof
oneormorechapters. SpecialthankstoEmielwithwhomIhadthepleasureof
co-authoringtwopaperswhichformthebuildingblocksofchapter3. Icannot
imaginehowIcouldhavewrittenthisdissertationwithouthavingbeenexposed
toEmiel’spositiveandenergeticapproachtodoingresearch.Furthermore,Irecall
withpleasurethelongphilosophicaldiscussionswithRene´inthetrainbetween
Tilburg and Eindhoven (one should not underestimate the time itcan take the
DutchrailwaystogetfromTilburgtoEindhoven). Iconsidermyselfveryfortu-
natetohavehadtheopportunitytobenefitfromKees’abilitytospottheweak
pointsofatheory.Hiscomments,thoughoftencritical,werealwaysconstructive.
Iwouldliketothankthefollowingpeopleforaskingpertinentquestionsand
making useful suggestions atsomepoint during the preparation ofthis disser-
tation: NicholasAsher,HansKamp,LeenKievit,PeterKrause,SteveBerman,
BarbaraPartee,RemkoScha,MarcSwerts,JacquesTerken,JanvanKuppevelt,
MargrietVerlinden, HenkZeevat,EdeZimmermannandeverybodyIforgotto
mention. Furthermore, thanks are due to all the people at IPOand the Com-
putationalLinguisticsGroupatTilburgUniversitywhomadethoseplacessuch
pleasantenvironmentstoworkin.
Theresearchreportedinthisdissertationwascarriedoutforthelargestpart
attheCentreforResearchonUser-SystemInteraction (IPO)oftheEindhoven
UniversityofTechnology,andforasmallerpartattheComputationalLinguistics
Group of Tilburg University. MyPhD.project was part of the DENK project
(DialogueManagementandKnowledgeTransfer). DENKwasfinancedbythe
Co-operationCentreofTilburgandEindhovenUniversities(SOBU).
Finally,thankstoEric,Erwin,BertandDimitriforhelpingmetogetmymind
4 AnswersandContexts 63
4.1 TheRoleofContext. . . . . . . . . . . . . . . . . . . . . . . . . 64
4.2 RepresentingquestionsandanswersinPTS . . . . . . . . . . . . 65
4.3 Formalizinganswerhood . . . . . . . . . . . . . . . . . . . . . . 68
4.4 FurtherPragmaticsofQuestionsandAnswers . . . . . . . . . . . 77
Table of Contents 4.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
5 AccentsandAlternatives 83
5.1 PresuppositionandPlurality . . . . . . . . . . . . . . . . . . . . 86
5.2 AlternativeAssertions. . . . . . . . . . . . . . . . . . . . . . . . 88
5.3 FurtherApplications . . . . . . . . . . . . . . . . . . . . . . . . 100
5.4 AlternativeProposals . . . . . . . . . . . . . . . . . . . . . . . . 108
5.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
Introduction xi
III Conversation 115
I Logic xxvii
6 ConversationalGames 117
1 ContextandConsistency 1 6.1 ConversationandDiscourseAnalysis . . . . . . . . . . . . . . . 118
1.1 ContextandInterpretation . . . . . . . . . . . . . . . . . . . . . 2 6.2 StoresandRules . . . . . . . . . . . . . . . . . . . . . . . . . . 123
1.2 ConsistencyandLogic . . . . . . . . . . . . . . . . . . . . . . . 8 6.3 AConversationalGame. . . . . . . . . . . . . . . . . . . . . . . 125
6.4 Applications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
2 PureTypeSystems 11
6.5 Comparisons . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
2.1 InformalExposition . . . . . . . . . . . . . . . . . . . . . . . . . 12
6.6 PresuppositionsinConversation . . . . . . . . . . . . . . . . . . 140
2.2 FormalExposition . . . . . . . . . . . . . . . . . . . . . . . . . 16
6.7 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
2.3 ContextsofInterpretation . . . . . . . . . . . . . . . . . . . . . . 19
2.4 ReasoningandNormality . . . . . . . . . . . . . . . . . . . . . . 20 7 ConveyedMeaning 145
7.1 InDefenseofLiteralMeaning . . . . . . . . . . . . . . . . . . . 146
7.2 SomeObservations . . . . . . . . . . . . . . . . . . . . . . . . . 148
II Information 27
7.3 TheConversationalGame. . . . . . . . . . . . . . . . . . . . . . 150
7.4 Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
3 PresuppositionsandProofs 29
7.5 Comparisons . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
3.1 PresuppositionsasGaps . . . . . . . . . . . . . . . . . . . . . . 32
7.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
3.2 ConditionalsandDisjunctions . . . . . . . . . . . . . . . . . . . 34
3.3 Bridging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
SummaryandConclusions 157
3.4 ConstructingDeterminateBridges . . . . . . . . . . . . . . . . . 44
3.5 RelatedWork . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 SummaryinDutch 161
3.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
3.7 Appendix:TheResolutionAlgorithm . . . . . . . . . . . . . . . 57 Notes 163
3.8 Appendix:TheQuestionnaire. . . . . . . . . . . . . . . . . . . . 60
References 169
Conversationanalysts(e.g.,Sudnow,1972)anddiscourseanalysts(e.g.,Sten-
stro¨m,1994)haveexaminednaturallyoccurringconversationsandprovidedde-
taileddescriptionsofthepatternsthatoccurinsuchconversations.Thesedescrip-
tions aremainlyintermsofanon-technical vocabulary thatisdirectly related
totheeverdaywaysofspeaking aboutconversation. Thedescription ofapat-
Introduction ternmayuseavocabularyconsistingofthetermssuchas‘question’,‘answer’,
‘request’,etc.
Althoughthedescriptionsthatarearrivedatinthiswayarevalid,theyrely
heavily on the judgements of the person that analysed the conversation. This
informerlabelscertainutterancesasquestions,othersasanswers,etc.Indoingso,
Theaimofthisbookistocontributetothedevelopmentofatheoryofconver- heorsheuseshisorherimplicitunderstandingoftheutterancesathand.Amodel
sation. Onthesurface, aconversation isasequence of(possibly overlapping) intermsofagameisintended toprovideanexplicit basisforsuchhighlevel
utterances. Weareinterestedinthequestionwhatmakessuchasequenceintoa descriptions. Thisisachievedbyrelatingthemove—i.e.,thestatetransition—
conversation?Thisquestionisapproachedbylikeningtheutterancesofaconver- thatanutterancegivesrisetodirectlyitssyntacticandprosodicsurfaceform.
sationtothemovesinagame.Thus,weask:whatkindofagameisaconversa- Inadditiontotheaforementionednaturallyoccurringconversations,constructed
tion? Wittgenstein(1953)wasoneofthemostprominentproponentsoftheidea fragmentsofconversationareusefulasameansfortestingthepredictionsofa
thatlanguageuseisagame-likeactivity. theory. Theadequacyofthetheorycanbeputtothetestbycheckingwhether
Agamecanbecharacterizedintermsofthestatesofthegameandtherules theanalysisthatthetheoryassignstosomefictitiousstretchofconversationcor-
thatregulatethetransitionsbetweenthestatesofthegame. Therefore,inorder responds with the judgements of ahuman informer. Ideally, such judgements
tomodelconversationasagame,henceforthaconversationalgame,weneedto shouldbeelicitedthroughquestionswhichdirectlymatchtheeverydayconcepts
specifythestatesandstatedynamicsofthisconversationalgame(cf. Hamblin, ofthenaiveinformer(see,inparticular,section3.4).
1971;Stalnaker,1978andLewis,1979). InpartIofthisbook,weintroducethe Thesortofadequacythatwehavediscusseduptillnowisindifferentwith
formaltoolstodoso.PartIIdealswiththestatechangesthatindividualutterances respecttotherelationbetweentheconversationalgameandthementalstatesof
giveriseto. Finally,inPartIIIwefocusontheinfluenceofconversationalrules thepersonsthatareinvolvedinaconversation.Sincenaturallyoccurringconver-
onsequencesofutterances. sationsinvolverealpeople,itislegitimatetoinquireintotherelationbetweena
Beforeprovidingasummaryofeachoftheaforementionedparts,wedescribe conversationalgameandthementalstatesoftheinterlocutors.
theassumptionsunderlyingourmodelofconversations. Wedealwithissuesof Togetthingsintoperspective,considertherelationbetweenadescriptionof
adequacyofaconversationalmodelandintroducelogicasatoolformodelling thegameofchessandthementalstatesofsomeindividual thatisplaying the
conversations. Furthermore,weindicatewhatpartofthecontextinwhichcon- game. Thedescription of thegame(the board, the chess pieces and the legal
versationsoccuriscoveredbytheconversationalstate.Finally,weprovidesome moves)neednotsayanythingaboutthewayaplayerrepresentsthisgameinter-
backgroundaboutthenotionofinformationthatisusedthroughoutthisbook. nally. Humanplayersshould,however,beabletorepresentthegamesomehow.
Similarly, itmakesnosensetoprovide adescription ofaconversational game
Adequacy whichcannotbeplayedbyhumanbeings.
Forpracticalpurposes,adescriptionoftheconversationalgamewhichallows
Theadequacyofaconversationalgameasamodelofconversationsdependson
a computer programme to play the gameis attractive. It makes it possible to
howwellitaccountsforthepatternsthatoccurinnaturallyoccurringconversa-
explorethepatternsthatareaccountedforbytheconversationalgame. Theex-
tions.Thesepatternsarethedatathatthegameistoexplain.Thepatternsthatwe
plorationcanbecarriedoutbyhavingthecomputerprogrammeplaywithacopy
takeasabasiswillbederivedfrombothnaturallyoccurringandconstructedcon-
ofitself(see,e.g.,Power,1979)orwithahumanbeing. Themodeldescribedin
versations,wherethelatterareintendedasconversationsthatcouldhaveoccurred
thisbookhasbeendevelopedpartlyonthebasisofourexperienceswithanim-
naturally.
xi xii
plementationofaconversationalgame. Thisimplementationwascarriedoutas compatible withtheinformational content oftheconversational state. Wedis-
partoftheDENKproject(seeAhnetal.,1995;Buntetal.,1998). cussthisroleofconversationalstatesindetailinpartIII.There,wedistinguish
betweenthecompabilityofphysicalactionsversuscommunicativeactionswith
LogicasaTool theconversationalstate. Forinstance,weproposethatiftheconversationalstate
containstheinformationthatoneoftheinterlocutorsisgoingtoperformsomeac-
Inordertoprovideanadequatemodelofconversations,weneedthetoolstocon- tion,thentheconversationalgameprescribesthatheshouldindeedperformthat
structsuchamodel. Whatrequirementsdoesthenatureofconversationsimpose action(e.g.,afteraninterlocutorhassaidthatheisgoingtothecinema,thisac-
uponsuchatool? Primarily,conversationsinvolvetheexchangeofinformation. tionisrecordedontheconversationalstateasacommitmentoftheinterlocutor).
Theconversationalstatekeepstrackoftheinformationthathasbeenexchanged Thecompatabilityofcommunicativeactionswiththeconversationalstateisex-
inthecourseofaconversationandtheinformationthatwasalreadypublicbefore plicatedprimarilyintermsofconsistency. Forinstance, thepresuppositions of
theconversationstarted. acommunicativeactionshouldbeconsistentwiththeinformationthatisalready
Inthecourseofthisbook,wewillshowthatmuchoflanguagebehaviourcan partoftheconversationalstate(cf.Stalnaker,1978;chapter3,thisbook).
beexplainedintermsofthequestforconsistentinformationbyconversational Inlinewiththeaforementionedviewontheconversationalstate,andfollow-
partners.Inordertostriveforconsistency,itisnecessarytodisinguishconsistent ingHamblin(1970),weusethetermcommitmentsfortheinformationalitemsthat
frominconsistentinformation. Sinceaninconsistencymightbeimplicitinsome makeuptheconversationalstate.InpartIII,weproposethatifaninterlocutorA
bodyofinformation,atoolisneededforbringinginconsistenciesintotheopen. assertssomesentenceSwhichexpressestheproposition ,thenitwillbecome
Atthispoint,logicentersthepicture.Logic—thestudyofcorrectreasoning— apubliccommitmentthatAiscommittedto . Let beth(cid:0)econversationalstate
providesthemeanstobringtolighttheinconsistenciesthatmightbehiddenina aftertheassertion,and meansAiscom(cid:0)mitted(cid:3)to . Inthatcase,wecan
bodyofinformation,suchasaconversationalstate.Givenabodyofinformation, write tostateCtAh(cid:0)at holdsintheconvers(cid:0)ational state . Wecall
ittellsushowtosetupachainofinferenceswhichleadsuptosomepieceof t(cid:3)he(cid:2)utCteAra(cid:0)ncemeaningaCnAd(cid:0) thesentencemeaningoftheafore(cid:3)mentioned
informationthatwasimplicitinthebodyofinformation.Forinstance,themodus CutAte(cid:0)ranceofthesentenceS. (cid:0)
ponensruleoflogicsaysthatonthebasisof and implies ,wemayinfer Atthispoint,letusaddresstherelationbetweentheterm‘commitment’and
that .Thefollowingargument,whichreveals(cid:0)theimp(cid:0)licitinform(cid:2)ationthat‘One sucheverydaywordsas‘belief’and‘intention’. Shouldweequatecommitment
cann(cid:2)otdriveinMike’scar’,islicensedbythemodusponensrule: withwhatwenormallycallbelief?Inotherwords,shouldtheutterancemeaning
beequivalentto (i.e.,Abelievesthat )? Moore’sParadoxappears
(1) Ifthemotorofacarisbroken,thenonecannotdriveinit. Themotorof
CtoAp(cid:0)rovidesomeevidenBcAe(cid:0)forthatposition. Thep(cid:0)aradoxinvolvesthefollowing
Mike’scarisbroken.
sentence:
THEREFOREOnecannotdriveinMike’scar.
Now,iftherulesoflogicallowustoinfersomepieceofinformationandits (2) Itrains,butIdonotbelievethatitrains.
negationfromabodyofinformation,thenthatbodyofinformationisinconsistent.
Thatis,asentenceoftheform ,butIdonotbelievethat . Moorepoints
outthatalthoughthesentencemean(cid:0)ingisnotself-contradictory(cid:0),theutteranceas
ConversationalStatesandContext
awholedoesseemtobeself-contradictory.Wecanexplainthisbyassumingthat
Inthechapterstocome,thenotionsofacontextandaconversational stateare istheutterancemeaningofadeclarativesentencewithasentencemeaning
usedinterchangeably. Thetermcontextis,however,oftenusedinamuchwider BAa(cid:0)ndaspeakerA.Theutterancemeaning of(2)(where isthe
sensethantheonethatweemploy.Inthissection,wepointoutinwhichrespects (cid:0)propositionitrains)isindeedself-contradicBtoAr(cid:4)yp,g(cid:3)iv(cid:4)eBnAsopm(cid:5)eplausibleaxipomsfor
ournotionofcontextismorerestrictedthantheusualone. belief(seeThijsse,1994;1998).
Thijsse(1994;1998), however,pointsoutthattheformalisationofutterance
Commitment Inourview,theconversationalstateorcontextdirectlyconstrains meaning in terms of is deficient. In particular, it cannot account for the
thebehaviouroftheinterlocutors: theyaresupposedtostriveforactionsthatare anomalyof‘Itrains,bBuAtIdonotknowthatitrains’. Aftercarefulanalysisof
xiii xiv
exampleslikesthelatter,Thijsseproposesthattheutterancemeaningofadeclar- ground,cf. Clark,1996),ifsomepieceofinformationiscommonground,then
ativesentencewithcontent andspeakerAis (where standsfor itisalsocommongroundthatitiscommonground. Furthermore,itwillalsobe
Abelievestoknow.Itmayb(cid:0)eparaphrasedas‘ABAisKcAo(cid:0)nvincedthBaAt’K).AThisfinding commongroundthatitiscommongroundthatitiscommonground,andsoon.
canbeincorporatedintoourframeworkbyaddinganaxiomto whichsaysthat This property of the common ground is appropriate for the conversational
foranyproposition , isequivalentto . (cid:3) state. Theconversationalstateisupdatedinthecourseofaconversation. After
Interestingly,the(cid:0)reCaAre(cid:0)alsoutterancesinBvAolKviAn(cid:0)gtheverb‘intend’whichap- thefollowingexchange:‘A:TheearthisflatB:Yes,sheis’,wemightaskthein-
peartobepragmaticallyanomalous.Consider,forinstance: terlocutorswhichinformationhasbeenexchangedasaresultoftheconversation.
Wecontendthattheanswerwouldbethatithasbecomepublic(amongstAand
(3) Iwillopenthedoor,butIdonotintendtodoit. B)thattheearthisflat. Whatismore,ithasalsobecomepublicthatitispublic
thattheearthisflat(andsoon).
Onthebasisofexample(3)wemightbetemptedtoconcludethataperson Thereisanotherpropertyofthecommongroundwhichturnsouttobecrucial
intendstodosomethingifheorshebelievesthatheorshewilldoit.Thiswould forthemodellingofconversationalstructures(seechapter6).Thepropertyisthat
giveusahandletoexplaintheanomalyof(3).Compare,however: ifitiscommongroundthatAiscommittedto andthatBiscommittedto ,
thenitiscommongroundamongstAandBthat (cid:0).Inotherwords,ifitiscommo(cid:0)n
(4) Iwillloosethematch,but/althoughIdonotintendtolooseit. groundthatAandBsharetheinformation ,th(cid:0)en iscommonground.
Formodellingthestructureofconvers(cid:0)ations,t(cid:0)hefirstmentionedpropertyof
Ifintentionandbeliefinafutureactionwereindeedthesame,then(4)would
thecommongroundplaysonlyaminorrole. Onthebasisofthefirstandthe
havetobeanomalous. Theutterancemeaningwouldthenfitthepattern
secondpropertywecan,forinstance,predictthatafterAhassaidthattheearthis
fromwhichacontradictioncanbederived. ItseemsthatforanaBgAe(cid:4)npt(cid:3)to
flat,itisuninformativewithrespecttotheconversationalstateforBtosaythatA
(cid:4)inBteAndp(cid:5)todosomething,itisnotsufficientthattheagentbelievesthathewilldo
iscommittedtotheearth’sflatness.
it.Weproposethefollowinganalysis: foranagenttointendanaction,heorshe
Clark(1996)distinguishesbetweenthecommunalandthepersonalcommon
shouldnotonlybelieveinperformingtheaction,butalsohavecontroloverthe
ground. Whereasthelatterisdirectlybasedonjointperceptualexperiences,the
action.
communalcommongroundconsistsofinformationthatpeoplesharebyvirtueof
Theanomalyof(3)nowfollowsfromthefactthatpeoplearenormallyas-
beingpartofaculturalcommunity.Communitiescanbebasedon,amongstother
sumedtobeincontrolofwhethertheywillorwillnotopenadoor.Therefore,the
things,language,nationality,profession,religionandhobbies.
negationof‘Iintendtoopenthedoor’—whichweanalyseasIbelieveIwillopen
Althoughconversationsalwaystakeplaceagainstthebackgroundofacom-
thedoorandIhavecontrolovermyselfopeningthedoor’—canonlymeanthat
munalcommonground,itisthepersonalcommongroundwhichisdirectlyacted
‘IdonotbelievethatIwillopenthedoor’,whichgivesusanutterancemeaning
(cid:0) uponinaconversation. Whereasthepersonal commonground changes inthe
of(3)whichisequivalenttotheutterancemeaningofMoore’sexample.
courseofaconversation,thecommunalcommongroundisnormallystaticwithin
For(4),thedenialoftheintentioncomesdowntothedenialofbeingincon-
theboundsofaconversation. Thedistinctionbetweenthedynamicandthestatic
trol,nottothedenialofthebeliefthatonewillloose. Thusweget: Iamcon-
(cid:2) partofthecontextcanalsobefoundinBunt(1994),whospeaksofthelocaland
vincedofthefollowing:Iwillloosethematch,andIbelievethatIwilllooseitand
theglobalcontext,respectively.
thatIhavenocontroloverwhetherIlooseit. Inthiscase,noinconsistencycan
Wedonottakethecommongroundtobeanentitythatexistsindependently
bederivedonthebasisoftheutterancemeaning.Thisisinlinewiththeintuition
oftheindividual language users. Thismeansthat whenwedescribe therules
thatthesentencecanbeutteredfelicitously.
andstatesofaconversational game,weshouldbearinmindthataplayerina
conversationalgamewillalwaysactonthebasisofhisorherrepresentationof
CommonGround InlinewiththepioneeringworkofStalnaker(1974;1978) theconversationalstate. Thisallowsforsituationswheredifferentplayershave
andLewis(1979),wetaketheconversationalstateorcontexttoconsistofthein- differentrepresentationsoftheconversationalstate.
formationthatispublictotheconversationalpartners,i.e.,theircommonground.
Intuitively(andaccordingtothebestknownformalizationsofthetermcommon
xv xvi
ContextinDENK Wetaketheconversationalstateorcontexttoconsistofpub- cupboard, thenBsaysso. TheDENK prototype employsprecisely thattactic.
licinformation,whereeachinformationalitemrepresentsapubliccommitment. However,othertacticsareconceivable. IfBknowsthatthereisonlyonecookie
Thisnotionofcontextissufficientfordefiningaconversationalgame,butfalls leftandwantstoeatit,Bmightanswer‘no’althoughBknowsthattherestillisa
short when it comes to specifying how an agent could play such a game. A cookie.
fullsystemforplayingconversationalgameshasbeenimplementedintheDENK Notethatinthelattersituation,Bisstilladheringtotheconversationalrules.
project (Buntetal.,1998). ThisbookwaswrittenaspartoftheDENKproject. However,Bviolatesarulethatmightbesaidtobepresupposedbytheconver-
(cid:3)
Letusexplainhowthenotionofcontextthatwedescribeherefitsintothebroader sationalgame,i.e.,trytomakeyourcontributiononethatistrue. Grice(1975)
notionofcontextthatisemployedintheDENKproject. termedthisthemaximofQuality.
(cid:4)
TheDENKarchitectureisbasedonasimpleconversationalsituationinvolv- Ournotionofaconversationalstateexcludestheinformationthatisprivately
ingtwointerlocutorsandadomainofconversation.Thedomainoftheconversa- availabletotheinterlocutors. Furthermore,wedonotdistinguishbetweenpublic
tionconsistsof(1)thatpartoftheworldthattheconversationoftheinterlocutors information obtained through conversation and public information obtained by
isaboutand(2)thatpartoftheworldwhichtheycandirectlymanipulateand means of joint perceptual observations on the domain of conversation. In the
observe. Ideally,thepartoftheworldthatcanbetalkedaboutandthepartthat DENKsystem,observationsonthedomainareexplicitlydealtwith.
canbemanipulatedandobservedarethesame. Thisdoesnotmeanthatevery Thesetupwhichwehavedescribed—i.e.,twointerlocutorsandadomainof
aspectofthedomainthatcanbetalkedaboutcanalsobeobserveddirectly. For conversation—maybecalledthegeneralizedDENKsituation.TheDENKarchi-
instance,regularitiesintheworld(e.g.,everytimeevent occurs,event oc- tectureitselfwasconceivedofasanewparadigmforhuman-computerinteraction.
cursaswell),canbetalkedabout,butarenotaccessiblevei(cid:0)adirectobservaeti(cid:2)on:a Takeasituationinwhichausercandirectlyinteractwithsomeapplication.Now,
singleobservationcannotverifyastatementthatquantifiesuniversallyovertimes theDENKarchitectureisobtainedbyaddingacooperativeassistanttothissitu-
(thoughitmayfalsifyit). ation. Thisassistant,whichisasoftwareagent,isabletoconversewiththeuser
InDENK,thecontextisequatedwiththeconversationaldomain,therepre- abouttheapplicationanddirectlyinteractwiththeapplication.
sentationsthattheinterlocutorsmaintainofthedomainandthestateofthecon- Theconnection withthegeneralized DENK situation isobvious: theinter-
versation. Inadditiontowhatwecalltheconversationalstate,thiswidernotion locutorsaretheuserandthecooperativeassistantandthedomainofconversation
ofcontextinvolvesthedomainofconversationandtherepresentationthateach correspondstotheapplication. FortheDENKprototype,theapplicationconsists
interlocutorprivatelymaintainsofthisdomain.Whereastheconversationalstate ofthesimulationofanelectronmicroscope. Across-sectionofthemicroscope
determinesthelegalmovesthatareavailabletoaninterlocutor,theremainingcon- anditscontrolpanelaregraphicallydisplayedtotheuser. Theusercandirectly
textprovidestheinformationforchoosingaparticularmovefromtheavailableset manipulatethismicroscopeorengageinaconversationwiththecooperativeas-
oflegalones. sistantabouttheapplication. ThelanguageoftheuserisafragmentofEnglish.
Consider,forinstance,asituationinwhichinterlocutorAhasaskedBaques- Theassistanthandlesquestions,(indirect)requestsandassertions. Furthermore,
tion. Inthatcase,theconversationalstatewillrequirethatBreactsinanappro- theassistantisabletodirectlymanipulatetheapplication(inresponsetorequests
priatewaytothequestion,forinstance,byprovidingananswer.Furthermore,the fromtheuser)andobservethestateoftheapplication(inordertoanswerques-
answershouldbecompatiblewiththeinformationthatisalreadypartofthecon- tionsfromtheuser).
versationalstate(seechapters4and6fordetails). Inotherwords,alegalmove ThecooperativeassistantisimplementedinPROLOG.Itconsistsoffourmain
byBmightconsistofananswerthatiscompatiblewiththeconversationalstate. modules: asyntacticanalyserwhichusesagrammarwhichiswritteninanex-
WithintheseboundariesBstillhasalotoffreedomwithregardstodetermin- tensionofHPSG (Verlinden,forthcoming),asemanticinterpreter,whichbuilds
(cid:5)
ingtheactualanswer. SupposethatAasked‘Aretherestillanycookiesinthe upanunderspecifiedrepresentationofanutteranceandusescontextualinforma-
cupboard’.‘Yes’,‘no’and‘Idonotknow’wouldbelegalresponsesbyB.Which tiontofillintheunderspecifiedparts(Kievit,forthcoming),adialoguemanager,
ofthethreeistheactualanswermightbebasedonB’sprivateinformationabout whichcontrolstheinteractionprocess,anddeterminesthereactiononthebasis
thedomainofconversation.Bmightchoosetoprovideananswerthatislicensed ofarepresentationofthecontext(seethereferencesattheendofthischapter),
byB’sprivateinformation. Forinstance,ifBthinksthattherearecookiesinthe andfinallyamoduleforreasoningaboutthedomainandobservingit(seeAhn,
xvii xviii
forthcoming). mationalitemsareintroductionsoftheform .Now,whatinformationdoes
anintroduction representtoanagent?VIn(cid:6)otTherwords,whatisthemeaning
InformationandContext of totheVag(cid:6)enTt? Theanswerisstraightforward: theagenttakes to
meVan(cid:6)(sTtandfortheinformation)thatthereissomeobjectofthetype .V (cid:6)T
ThecooperativeassistantoftheDENKprojectmaintainsformalrepresentations Thisanswercanbedevelopedindifferentdirections. TheagentcTan(cid:6)consider
ofthecontext. Theserepresentationsbelongtoaclassofformalsystemsknown the type to be part of a scheme ofclassication of reality, i.e., ‘the external
asPureTypeSystems(PTS;Barendregt,1992). PTSisageneralizationoftyped world’. ITnthat case, means to the agent that there exists an object in
lambdacalculus(Curry,1943;Church,1940). Inparticular,inDENKtheinfor- theexternalworldwhiVch(cid:6)fitTsthetype . Alternatively, theagentmightuse
mationalcontentoftheconversationalstate,whichisdealtwithinthisbook,is toclassifyitsexperiences. Inthatcase,T wouldmeantotheagentthataTn
modelledinPTS. experienceoftype occurred. V (cid:6) T
Incidentally,theformalobjectsthatPTSprovidesformodellingconversational ThelatterapproTachhas,however,seriouslimitationswhenitcomestomod-
statesarecalledcontexts. SuchaformalcontextinaPTSisasequenceofintro- ellingcommunicationbetweenagents. Whatwoulditmeanfortheinformation
ductionsofobjects.Eachoftheseintroductionsisoftheform ,where is of tobetransferredfromoneagenttotheother? Sinceexperiencesare
avariableand isthetypeofthevariable.Thevariable stanVds(cid:6)foTranarbitVrary privVate(cid:6)tTotheagents, willnevermeanthesamethingfortwodifferentagents.
objectoftypeT.Therefore,wewilloftenspeakofthe(aVrbitrary)object . Inparticular,ifanageTntisconvincedthatthemeaningof residesstrictly
APTSconTtextcontainsinformationinthefollowingsense: itrecordVswhich withitsownexperiences,thenitwillfinditimpossibletoVsha(cid:6)reTtheinformation
typesareinhabited,i.e.,forwhichtypesthereareobjectsbelongingtothosetypes. associatedwith withothers,sincetheseexperiencesarealientotheothers.
WecandologicinaPTSbecausepropositionscanbeseenastypes.Thisinsightis WeconcludeVth(cid:6)aTttheinformationthatisconveyedbyanintroduction
knownasthepropositionastypesinterpretationoftypedlambdacalculus(Curry toanagent(i.e.,accordingtotheagent’ssubjectivepointofview)andthVatc(cid:6)aTn
andFeys,1958). Propositionaltypesareinhabitedbyproofs. Inotherwords,if becommunicatedisobjectiveinnature;theinformationisaboutarealitythatis
thecontextintroducesanobjectasbelongingtoaparticularproposition,thenthis externaltotheagent. Paradoxically,ifwelookatthemeaningofanintroduction
objectshouldbeseenasstandingforaproofoftheproposition. fromaperspec(cid:7)tivethatisexternaltotheagent(i.e.,fromanobjectivepoint
PTSencompassesruleswhichdescribehowtheobjectsandthetypesthatare Vof(cid:6)viTew),thentheinformationassociatedwith becomessubjective. Letus
availableinacontextcanbeusedtoconstructnewobjectsandtypes. Consider explain. V (cid:6)T
acontext containingtheintroductions and . Inwords,accord- Wemustaskthequestion,‘Howdoesanagentobtaininformationaboutthe
ingtothe(cid:3)context thereisaproof forath(cid:6)eppropofsit(cid:6)iopn(cid:5)aqndaproof forthe world?’ormorespecifically‘Howdoesanagentacquiretheinformationthatthere
proposition (cid:3). InlinewithHeyating’sconstructivistepxplanationoffimplica- existsanobjectofsometype ?Asobjectiveobserversoftheagent,wemaysee
tion(Heytinpg,(cid:5)195q6),aprooffor correspondstoamethodforobtaining theagentinteractwiththeoutsTideworldandrecognizecertainobjectsasinstances
proofsof fromproofsof . Intper(cid:5)msqofaPTS,thisistranslatedinto: aproof of . Thisabilitytorecognize inhabitants ofatypeisentirelypersonaltothe
for qisafunctionfropmproofsof intoproofsof . Thus, canbeused ageTnt. Thus,fromanexternalpointofviewthemeaningof issubjective.
tocpon(cid:5)struqctaproofof fromtheproofpof byapplyinqg to ;fweobtainthe Theagentitself,however,cannottakethisstandwithoutbecomVin(cid:6)gTasolipsist,i.e.,
proof (‘’standsfqorfunctionapplicaationp)of . Thenoftatioanfor‘ isa withoutdenyingthepossibilityofcommunicationwithotheragents.
proofoff(cid:6)agiv(cid:6)encontext ’is: . q f(cid:6)a Wehaveused‘themeaningof ’and‘theinformationthat stands
WehqaveseenthatPT(cid:3)Sallo(cid:3)w(cid:2)usft(cid:6)oad(cid:6)oqcalculationswithintroductions: infor- for’interchangeably. Inourview,Vm(cid:6)eaTningandinformation areonVe(cid:6)oTfakind.
mationthatisimplicitinabodyofinformationcanbemadeexplicit.Letusnow Thereisnodirectrelationbetweenthisapproachtoinformationandthenotionof
addressthequestion‘Inwhatsensedoesanintroduction representapiece informationthatisusedininformationtheory(Shannon,1948).
ofinformation?’ Forthatpurpose,wehavetogobeyondVth(cid:6)eTboundariesofthe Informationtheoryisconcernedwiththetransmissionofmessagesfroman
formalsystemofPTS. informationsourcetoadestination. Informationisthoughtofasachoiceofone
Ourmainpremissisthatinformationdoesnotexistindependentlyofanagent messagefromasetofpossiblemessages, eachofwhichisassociatedwiththe
towhomtheinformationismeaningful.Theformalobjectsforrepresentinginfor-
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