Semantics based on Success, Satisfaction, and
Recursion
BrahimChaib-draa 12
andDanielVanderveken 3
1
LavalUniversity,ComputerScienceDepartment
PavillonPouliot,Ste-Foy,PQ,Canada,G1K7P4
chaib@ift.ulaval.ca
2
DFKIGmbHStuhlsatzenhausweg3,D-66123Saarbrucken,Germany
3
DepartementdePhilosophie,PavillonRinguet,CP500
UQTR,Trois-Rivieres,PQ,CanadaG9A5H7
DanielVanderveken@uqtr.uquebec.ca
Abstract. SearleandVanderveken'smodelofspeechactsisundoubt-
edlyanadequatemodelforthedesignofcommunicatingagentsbecause
itoersarichtheorywhichcangiveimportantpropertiesofprotocols
thatwecanformalizeproperly.Weexaminethistheorybyfocusingon
thetwofundamentalsnotions,success andsatisfaction,whichrepresent
asystematic, uniedaccount of both the truthand thesuccess condi-
tional aspects. Then, we propose an adequate formalism{thesituation
calculus{forrepresenting thesetwonotions (ina recursive way)inthe
context of agent communication language. Theresulting framework is
nallyusedfor(1)theanalysisandinterpretationofspeechacts;(2)the
semanticsanddescriptions ofagent communicationlanguages.
1 Introduction
Speechacttheorydealswithnaturallanguageutterances.Infact,itwasdevel-
oped todealwith utteranceslike\I declareopentheconference",that arenot
easyclassiedasbeingtrueorfalse,butratherareactions.Lateritwasextended
to dealwith allutterances,with theprimary visionthat allutterances areac-
tionsofsomesortortheother[2,3,17].IndistributedAIandmorespecicallyin
multiagentsystems,oneofthemostnaturalwaysinwhichintelligentinteraction
mayoccur is throughcommunication, especially communicationabout action.
Agentsmaycommand,request,advise,report,orpermiteachothertodocertain
actions.Theymayalsopromiseactionsoftheirown,orprohibitthoseofothers.
Therefore, speech act theory can be considered asa foundation forcommuni-
cation among agents. In this paper, we present a new semantics for this kind
of communicationbased on success and satisfaction; twonotions which repre-
sentasystematic,uniedaccountofboththetruthandthesuccessconditional
the Truth and the Success Conditional Aspects
Linguistshavelongacknowledgedtheillocutionaryaspectsofsentencemeaning
in their analysis of the dierent syntactic types of sentences in natural lan-
guage. Using speech act theory, one can reformulate as follows their analysis
of themeaningofmostcurrenttypesof sentences [18,20,21]:representatives or
assertives which represents a state of aairs, e.g. statements; directives which
askthehearertodosomething,e.g.orders;commissiveswhichleadthespeaker
tocommitherselftodoingsomething,e.g.promises;expressiveswhichexpressa
certainpsychologicalstate,e.g.congratulations;declarativeswhichbringsome-
thingaboutintheworld,e.g.anexcommunication.
Mostelementaryillocutionaryactsthatarethemeaningofsentencesinacon-
textofutteranceconsistofanillocutionaryforcef togetherwithapropositional
content p(an elementary act will be therefore represented by hf;pi) [18]. For
example,thetwoutterances \Youwill leavetheparty"and \Leavetheparty!"
havethesamepropositionalcontent,namelythat youwill leavetheparty;but
therstofthesehastheillocutionaryforceofanassertionaboutthefutureand
thesecondhastheillocutionaryforceofanorder.
In order to analyze the logical form of illocutionary acts, one must dene
recursively their conditionsof success and of satisfaction, aswellas the truth
conditions of their propositional content. The conditions of success of an illo-
cutionary act are the conditions that must be obtained in a possible context
of utterancein order that the speaker succeed in performing that act in that
context. For example, a condition of success of apromise is that the speaker
commithimselfincarryingoutafuturecourseofaction.Theconditionsof sat-
isfaction of an illocutionary actare the conditionsthat must be obtainedin a
possible context. Forinstance, acondition of satisfactionof apromise is that
thespeakercarriesoutin theworldthe future courseof actionrepresentedby
thepropositionalcontent.
Infact,inspeechacttheory thenotionofsatisfactionisageneralizationof
thenotionoftruththat isneededtocoverallpossibleillocutionaryforces.Just
as anassertionissatisedifandonlyif(i)it istrue,apromiseissatisedi
it is kept, a request is satised i it is granted,a question is satised i it is
answered, etc. Incaseof satisfaction ofan illocutionary act, there is asuccess
oft betweenlanguageand theworld.Thepropositionalcontentis truein the
sensethat itrepresentsanactualstateofaairsintheworldofutterance.
According to Searle and Vanderveken [18], each illocutionary force can be
dividedinto:(1)anillocutionarypoint,(2)amodeofachievementofthatpoint,
(3)propositionalcontent,(4)preparatoryand(5)sincerityconditionsandnally,
(6)degreeofstrength.Wewillnowexplainthenatureofthesecomponents.
1. TheIllocutionaryPoint.Illocutionarypointistheprincipal componentofan
illocutionaryforcef becauseitdeterminesthedirection oftof utterances
withthat force.Moreprecisely,itdetermineshowthepropositionalcontent
thespeechactcanbesatised.Aspointedoutelsewhere[18],therearefour
directionsof t and veand only ve illocutionary points ofutterances in
language:theassertive,commissive,directivedeclarative,andthe expressive
points.Thefourdirectionof tareasfollows
(i) Thewords-to-worlddirectionoft.Here,whentheillocutionaryacthf;pi
is satised, its propositional content ts a state of aairs existing in
the world. Speech acts with the assertive point such as, for example,
predictions,conjecturesandobjectionshavethewords-to-worlddirection
oft.Theirpointistorepresenthowthethingsarein theworld.
(ii) The world-to-words direction of t. When the illocutionary act hf;pi
is satised, the world is transformed to t the propositional content.
Speech acts with the commissive or directive point such as, promises,
recommendations, supplications and demands have the world-to-words
direction of t. Their point is to get the world to be transformed by
the future course of action of the speaker (commissives) or the hearer
(directives)inordertomatchthepropositionalcontentoftheutterance.
(iii) The double direction of t. Here, when the illocutionary act hf;pi is
satised,theworldistransformedbyanactionofthespeakertotthe
propositionalcontentbythefactthatthespeakerrepresentsitasbeing
sotransformed.Speechactswiththedeclarativeillocutionarypointsuch
asforexample,actsofappointing,nominatingand endorsing,havethe
double direction of t. Their point is to get the world to match the
propositional contentby sayingthat thepropositional contentmatches
theworld.
(iv) Theemptydirection oft.Forsomeillocutionaryacts,thereisnoques-
tion of success or failure of t, and their propositional content is in
general presupposed to be true. Speech acts with the expressive point
such as for example, apologies, thanks, congratulations, etc., have the
emptydirectionoft.Theirpointistoexpressapropositionalattitude
ofthespeakeraboutthestateofaairsrepresentedbythepropositional
content.Thepointhereisnottorepresentthatstateofaairsasactual
ortotrytogetittobeactualintheworld.
2. ModeofAchievement.Mostpurposesofouractions,andparticularlyillocu-
tionarypoints, canbeachievedin various waysorbydierentmeans.The
modeofachievementofanillocutionaryforcedetermineshowitspointmust
beachievedonthepropositionalcontentincaseofsuccessfulperformanceof
anactwiththatforce.Forexample,inacommandthespeakermustinvoke
apositionofauthorityoverthehearerandin arequesthemustgiveoption
ofrefusaltotheaddressee.Themodesofachievementofillocutionaryforces
preciselystatetheconditionsofachievementoftheirpoint.InEnglish,they
areexpressedbyadverbssuchasforexample\surely"and\whetheryoulike
itit or not" which modify the verb in sentences such as (a) \Surely, he is
here"and (b) \Whether you likeit ornot,do it!".Special other modes of
achievementinclude\humbly"and\politely"whichmodifytheperformative
onthesetofpropositionsthatcanbetakenaspropositionalcontentsofacts
with that force in a context of utterance. For example, the propositional
contentofapromisemustrepresentaspeaker'sfuturecourseofaction.The
propositional content ofa report must representastate of aairswhich is
eitherpastorpresentwithrespecttothemomentofutterance,etc.
4. Preparatory Conditions.Wheneveraspeakerattempts to performan illocu-
tionaryact, hebelievesthat certain propositions hold in thecontext of his
utterance.Forexample,aspeakerwhopromisestodosomethingpresupposes
thathis futureactionpromotesthehearer'sutility.Thepreparatorycondi-
tionsof an illocutionary force f determine which propositions the speaker
believes if he were performing an act with that force and a propositional
contentpin apossiblecontextofutterance.
5. SincerityConditions.Ofcourse,byperforminganillocutionaryact,thespeaker
alsoexpressesmental statesofcertain psychologicalmodesaboutthestate
ofaairs representedbythe propositional content. Forexample, aspeaker
whopromisessomethingexpressesanintentiontodowhathepromises,and
aspeakerwhorequestsahearerto dosomethingexpressesadesirethat he
do it. As in the case of propositional content and preparatory conditions,
somesincerityconditionsaredeterminedbytheillocutionarypoint.Forex-
ample,allassertiveillocutionaryforceshavethesincerityconditionthatthe
speakerbelievesthepropositionalcontent.
6. DegreeofStrength.Evidently,thementalstateswhichenterintothesincerity
conditionsofspeechactsareexpressedwithdierentdegreesofstrengthde-
pendingontheillocutionaryforce.Forexample,thedegreeofstrengthofthe
sincerityconditionsofasupplicationisgreaterthanthatofrequest,because
aspeaker,whosupplicates,expressesastrongerdesirethanaspeakerwho
requests.Degreeofstrengthisingeneralorallyexpressedbytheintonation
contourin English. Precisely, an increase in the degree of strength of the
intonationcontour servesin general to increasethe degreeof the sincerity
conditions.Adverbslike\sincerely" alsoservesto strengthenthe degreeof
strength of the sincerity conditionin sentences such as \I sincerely advise
youtodoit".
3 The Situation Calculus: A Logical Formalism for
Reasoning About Knowledge and Action
The situation calculus [10] seems to be an adequate formalism for reasoning
aboutactionsandtheireectsontheworld.Axiomsareusedtospecifythepre-
requisitesofactionsaswellastheireects.Recently,Reiter[14]hasgivenasetof
conditionsunderwhichtheexplicitspecicationofframeaxiomscanbeavoided.
Thissolutionisextendedtotheframeproblemtocoverknowledge-producingac-
tions,thatis,actionswhoseeectsaretochangeastateofknowledge[15].Notice
thatReiter'sapproachdoesnot,addresstheramicationproblem,hisapproach
which monotonicallysolves someversions ofthe ramication and qualication
problems.
With situation calculus,we canalso encodemessageswhichdepend onthe
words and the situation in which the words are uttered. In this perspective,
just asinsituation calculus,theencodinganddecodingfunctionstakeanextra
argumentrepresentingthecurrentsituation.
For these reasons, we pursue the perspective of situation calculus for rea-
soningaboutactionsin multiagentsystems,and speciallythe reasoningabout
speechactsforthecommunicationbetweenagents.Toachievethis,weincorpo-
ratesemanticsofintensionallogicandillocutionarylogicinthesituation calcu-
lus. Wewill nowpresentbriey the situation calculus, then we will show how
wewillusesuchcalculusforreasoningaboutknowledgeandaction.
In situation calculus, terms are used to represent states of the world -i.e.
situations.Ifisanactionandsasituation,theresultofperformingasituation
insisrepresentedbydo(;s).Poss(;s)meansthatitispossibletoperform
the action in the situation s. The constant S
0
is used to denote the initial
situation; and there is also anordering relation on situations , where s 0
s
standsfor \s 0
canbereachedfrom sby asequenceofoneormoreactions".A
uent F isa function dened onsituations. Forinstance, in theblocks world,
thelocationofagivenblockxisauentwhosevaluearethepossiblelocations
of blocks. Inthe languageof the situation calculus,the value of this uent at
s forablockx is denotedby location(x;s).Moregenerally,F(do(;s))means
thatF becomestrueinthesuccessorsituationdo(;s);and:F(do(;s))means
that F becomesfalsein thesamesuccessorsituation.
Beforewetreatepistemicuents,letusintroduceanalternativeformulation
in order to facilitate the logical formalismfor reasoning about knowledge and
action. Rather than introduce a situational argument to all of the predicates
in ourdomain, we caninstead reify predicates likeloc, making objectsout of
sentencessuchasloc(b;l).Moreprecisely,insteadofwritingloc(b;l;s)toindicate
thatthelocationofbislinsituations,wecanwriteloc(b;l)[s],whereloc(b;l)is
nowanobjectofourdomaininsteadofasentence.Whatloc(b;l)[s]saysisthat
theobjectloc(b;l)holdsin thesituations. Oneadvantageofreicationis that
itallowsustoquantifyoversentences(nowobjects)beingreied.Forinstance,
ifwewanttosaythatnothingholdsinsomesituations
1
,wecouldwritethisas:
8p :p[s
1 ]
If we have not adopted sentence reication in our domain, this axiom would
involve quantication over predicatesand would therefore not be alegitimate
sentenceofrst-orderlogic.
However,someatomicformulalikePossanddoarebinaryfunctions,whose
argumentsareanaction andasituationand in thiscaseitis importantto use
thereicationappropriately.Forinstance,ifweexpressbymove(x;y)theaction
ofplacingxontopofy,wecandescribetheeectofthisactionbythefollowing
asituation s 0
is understood asbeing accessibleforan agent ifrom asituation
sifasfarasibelieves(forexample)in situations, hemightbeinsituation s 0
.
Thus, somethingis believedin sif itis truein everys 0
accessiblefrom s, and
conversely something is not believed if it is false in some accessiblesituation.
Therefore,theusualbeliefoperatorcanbetreatedasanoperatorwhichreects
amental stateand which canhold ornotin somesituation s. Tothis end, we
introduceabinaryrelationB
i (s
0
;s),readas\s 0
isaccessibleforifroms".Thus
relationB
i (s
0
;s)holdsinsis 0
iscompatiblewithwhatibelievesins.Wecan
nowintroducetheobjectbel(i;p)readas\agentibelievesp"anddene itas:
bel(i;p)[s]
def
= 8s 0
B
i (s
0
;s)p[s 0
] (1)
Thisis the usual\knowledge"operatorand is considered hereasprimitive
modaloperator.Concerningthisoperator,weassumetheusualaxiomschemata
correspondingtoa\weakS5"modallogic.
Weassumeherethat goalis notaprimitiveatomic predicate,asin Cohen
and Levesque [5], because this predicate is based on the relation G
i
, which is
constrainedbyB
i
.Infact,therelationG
i
canbedenedastheintersectionofB
i
andanaccessibilityrelationexpressingthefundamentalnotionof\interest",i.e.,
thesetofsituationsthat anagentwouldwishweretrue.Wecall sucharelation
I
i
,andthecorrespondingpredicatewish.Inthese conditions,wish(i;p)means
\agentihasaninterestthatpistrue".Thispredicateisdenedbythefollowing:
wish(i;p)[s]
def
= 8s 0
I
i (s
0
;s)p[s 0
] (2)
As G
i
= B
i T
I
i
, this allows to us to introduce the \goal" predicate which is
dened by:
goal(i;p)[s]
def
= 8s 0
G
i (s
0
;s)p[s 0
] (3)
Nowitistimeto giveourformalizationofability.Infact,webase thisformal-
izationonthatofMoore[11],whichinspiteofitsrelativesimplicity,doesgetat
the essentialconnectionbetween theabilityof agentsto achievegoalsand the
knowledgetheyhaveaboutrelevantactions.Toformalizethisability, weintro-
ducerstlytwooperatorsforwhichsituationswillnotbereferredtoexplicitly:
res(a;p)and agt(i;a). Therstoperator res(a;p) will meanthat itis possible
fortheeventdenotedbyatooccurandthat,ititdid,theformulapwouldthen
betrue.ThesemanticsofthisoperatorissimilartotheoperatorRESofMoore
[11].The second operator agt(i;a)sayshere that agenti is theonly agentfor
theactiona.
Inthese conditions,theoperatorcan(i;a;p) read as\agenti can achievep
byperformingactiona" satisesthefollowing:
8i9xbel(i;(x=a)^agt(i;x)^res(a;p))[s]can(i;a;p)[s] (4)
This captures thefact that anagenti canachievepbyperforming acta ifhe
agenttoknowfromtheverybeginningofthisaction,particularlyifitiscomplex,
\what"sheisgoingtodoeverystep.
Wealsoneedaformaldenitionof\commitment".Forthispurpose,weaug-
mentourformalsystemwithanewaccessibilityrelationC
i
whichisEuclidean,
transitive, serial and such as C
i I
i T
B
i
. According to our intuition, C
i ac-
cessessituations which theagenti regardsasbothdesirableandpossible.The
situationsinC
i
areconsequentlythoseinwhichtheagentdoestheaction(s)that
shehasdecidedtodo.Nowwecandeneapredicatecmt(i;p) readas\agenti
iscommittedtoachievingp"anddeneitas:
cmt(i;p)[s]
def
= 8s 0
C
i (s
0
;s)p[s 0
] (5)
Anothermental stateisthe\intention"forwhichweneedaweaknotionofan
agenthavingaplan[13].Wenotethis versionofplan
has:plan(i;;p)
This means that i has the plan to achieve p. Notice that our weak notion
of havingaplan states:(1) i believesthat hecan executeeach actin ; (2) i
believesthatexecutingtheactsin will entailtheperformanceofpand,(3) i
believesthat eachactin playsarole.
Now,wecandene anewpredicateint(i;p) readas \theagenti intendsto
achievep"as:
int(i;p)[s]
def
= 9 cmt(i;p)[s]^has:plan(i;;p)[s] (6)
Howshould variousfeatureofintentionsfollowfrom previousdenitions?
1. Intentionsmustbeconsistent.Thismeansthatanagentcannotbecommit-
tedtotwosimultaneous conictingactions.Thisisthecasesincesituations
are internally consistent and according to the denition of cmt, the two
actionsmustoccurinallthesituationsinrelationbyC
i .
2. Intentionsarenot closedunder expectedconsequence. Thisfollowsfromthe
fact that we do not want cmt to be closed under (expected) implication.
Particularly,cmt(i;p) ^ bel(i;pq)doesnotimplycmt(i;q).
3. Intentions mustbe realistic. An agent mightbelieve that she will do some
actionwithouthavingaplan,that is,withouthavinganideaonhowtodo
it.Therefore,shecan haveacommitment,butnotanintention.
Finally,weneedtoformalize\Obligations"astheyareused,forinstance,in
thepromisestoreectthemodeofachievement.Obligationsrepresentwhatan
agentshoulddo,accordingtosomesetofnorms;itsformalaspectsaregenerally
examinedusingDeonticLogic(e.g.,[22]).
Obligationsaregenerallydierentfromandcannotbereducedtointentions
and goals,Thus, although knowingthat pis notcompatible withhergoals,an
In the case of obligations, an agent chooses to violate her obligations or not
dependingonthepricetopay.As arstapproximation,wecanexpressed\iis
obligedtoj tomakeptrue"byoblig(i;j;p)whichisdened bythefollowing:
oblig(i;j;p)[s]
def
= (9s
l
s)^(8s 0
:ss 0
s
l )
:p[s
l
]^whish(i;p)[s 0
]violating(i;j;p)[s
l
] (7)
Thus, if p is not achieved by i in some limit situation s
l
, i violates her
agreementbetweeniandjonp.Suchaviolationisrepresentedbythepredicate
violating(i;j;p).Evidently,betweensituationssands
l
,agenti hasaninterest
that pwould betrue.Westatethatbywhish(i;p)[s 0
].
Tosumup,theknowledgeandactionapproachdeveloppedhereisin facta
contributiontothebelief,desire,intention(BDI)model.Nowadays,itiswidely
acceptedthatthebahaviorofanyagentis mainlygovernedbythespecic way
ithandlestherationalbalancebetweenitsbeliefs,desires,andintentions.Other
papersinthis book refertotheBDImodel[7],[12],[16],[24].
4 Semanticsof Speech Actsbased on Success, Satisfaction
and Recursion
Theconditionofsuccessofanillocutionaryactaretheconditionsthat mustbe
obtainedin apossiblesituation(i.e.context)in orderthat thespeakersucceed
in performingthatact inthat situation.Forinstance, aconditionof successof
arequestisthatthespeakerattemptstogetthehearertocarryoutthefuture
courseofactionrepresentedbyp.
Moreover,communicationbetweenagentscanfailevenillocutionaryactsare
successfullyperformed.Inthiscase,theillocutionaryact(s)ofthiscommunica-
tionarenotsatised.Forexample,arequestwhichissuccessfullyperformedby
aspeakerissatised,onlyifthehearermakesitspropositionalcontenttrueby
carrying outin the multiagent environment thecourse of actionthat it repre-
sents.Moregenerally,theconditionsofsatisfactionofaspeechactcorresponds
to the conditions under which we would aÆrm that the given speech act has
beensatised.
Infact, conditionsof success and of satisfaction are a partof semanticsof
speechacts.AformalsemanticsisimportantforMASbecauseweneedarigor-
ousunderstandingofcommunicationinordertodesignandanalyzeamultiagent
systems. Tothis end, we propose in this section to capture conditions of suc-
cess and satisfactionin thesituation calculus, byusing the dierent operators
introducedintheprevioussection.Todothis,weadoptthefollowingSingh'sno-
tations[19]:(1)amessagemisapairhf;pi,wheref identiestheillocutionary
force,andptheproposition.Inthisnotation,f isanatomicsymbolfromtheset
fassertive,directive,commissive,declarativeandexpressiveg;and pis alogical
formula;(2)acommunicationfromitoj isrepresentedbycomm(i;j;m).Ifwe
consider says:to(i;j;m) as the only action that agent i can perform to make
We need also to express the psychological states that enter into sincerity
conditions which dierent degrees of strength depending on the illocutionary
force (seeSection 2). Forthis purpose, weuse integerswhich serveto measure
thedegreesofstrengthofillocutionaryforces.Byconventionweselectzero(0)
torepresenttheneutraldegree ofstrengththatischaracteristicoftheprimitive
illocutionary forces of utterances (such as assertion); +1 represents the next
stronger degree of strength (e.g. testimony); +2 the next stronger degree of
strength(e.g. solemnactsofswearingthat somethingisthecase).Similarly,-1
representsthegreatestdegreeof strengthsmallerthan 0(e.g. conjecture),and
soon. If degree()represents thedegree of strength of anact of type , then
degree()=k means that iftheact is performedin situation s, thespeaker
S expressespsychologicalstateswithdegreek.Withsuchadegreeof strength,
we can order the speech acts for each illocutionary force. Thus, the following
illustrates\some"degreesinthecaseofhdirective;pi:
degree(order)=degree(command)=degree(require)=+1
degree(ask)=degree(tell:to)=degree(request)=0
degree(suggest)=degree(advise)=degree(recommend)= 1
Wehavesimilarorders fortheassertives,commissives,declarativesandex-
pressives (details are in [21]). A such order between speech acts has in fact
manyimplicationsin cooperativesystemscommunication.Forinstance, anact
of request or ask type lets the Hearer H know that the Speaker S is either
of the samerankora lower rank.Thus, H can grantorrefuse therequest by
returningmessageswitheitheranassertoranswertype.Roleshaveasfunction
to reect the position ofeach agentin the hierarchyof the multiagentsystem
andtodeterminewhatreasoningstrategiestouse.Tocomparetherankingdif-
ferencesofagents,wecanassignanumberto everyroleasinCOSMO[23].For
example,fortwoagentsiofrole
i
andj ofrole
j
,agentirankshigherthanj i
v(role
i
)>v(role
j
)where v(role
x
)denotestherolevalueofanagentx.
4.1 The Conditionsof Success
AsSearleandVanderveken[18]pointedout,theconditionsofsuccessofelemen-
taryactsareuniquelydeterminedbythecomponentsoftheirillocutionaryforce
andbytheirpropositionalcontent.
Proposition 1: An illocutionary act of the form hf;pi is successfully
performedinthecontextofanutterance(s)itheconditionsofsuccess
ofhf;pi holdins.Formally,westatethisbythefollowing:
success(comm(S;H;hf;pi);s)cond:success(hf;pi)[s] (8)
Inthisformulation,success(comm(S;H;hf;pi);s)statesiftheacthf;pi
u
expressesifthe conditionsofsuccess ofhf;pi hold ornotin s. Finally,
s
u
standsforthesituationofutterance.
AsspeciedinSection2,theconditionsofsuccess(cond:success)for
anillocutionaryactoftheformhf;piwithrespecttosare:
1. the speaker achieves the illocutionary point of the force f on the
proposition p;
2. thespeakerachievesthisillocutionarypointwiththemodeofachieve-
mentoff;
3. psatisesthepropositional contentconditionsoff withrespecttos;
4. thespeakerpresupposesthepropositionsdeterminedbytheprepara-
toryconditionsoff;
5. thespeakerexpresses,with thedegree of strengthoff, thepsycho-
logicalstatesofthemodesdeterminedbythesincerityconditionsof
f aboutthestateofaairsrepresentedbyp.
Theseconditions canbe formulatedforanyspeech actsusing thesituation
calculusand thedierentoperators introduced in theprevioussection.Forex-
ample, a speaker S makes a promise for the hearer H in a context of ut-
terance (s) if and only if the following conditions (i.e. cond:success) hold in
s=do(says:to(S;H;hcommissive;pi);s
u ):
1. S commitsherselfto makeptrue(illocutionaryact).cmt(S;p)[s]
2. S isobligatedto H tomakeptrue(modeofachievement).oblig(S;H;p)[s]
3. Thepropositionalcontextoftheutteranceisthatthespeakerwilldo\some-
thing"tomakeptrue(propositionalcontentconditions).
9a9s 0
(s 0
s)bel(S;p)[do(a;s 0
)]
4. Spresupposesthatheiscapableofdoingsomeactionatomakeptrueand
thatpisin theinterestofH (preparatoryconditions);
9acan(S;a;p)[s]^bel(S;goal(H;p))[s]
5. S expresseswith thedegreeofstrength ofapromise,an intentionto make
ptrue(Sincerityconditionswithadegreeofstrength).
int(S;p)[s]^degree(promise)[s]
Wecan state similar conditions of success for hassertive;pi, hdirective;pi,
hdeclarative;pi,andhexpressive;pi.Forexample,SrequestsHaboutp,insome
contexti,inthiscontext:(1)SexpressesawishtogetHtocarryoutthefuture
courseofactionrepresentedbyp(illocutionarypoint);(2)inthiswhish,Sgives
a(more orless total)optionof refusal to H (mode of achievement);(3) phas
thegeneralpropositionalcontentconditionthat itscontentrepresentsafuture
course of action of H (condition on p); (4) S presupposes that the hearer is
capable of doing some action a to achieve p (preparatory condition); (5) and
nally,S expresseswithanormaldegreeofstrengthawishthatH dothatact
(sinceritycondition).
Similarconditions of success can be formulatedfor theother illocutionary
defective. Thus, a speakerS might actually succeed in asserting or promising
something even though he has not enough evidence for her assertion or her
promise might be insincere. In fact, an ideal speech act is one which is both
successfulandnondefective.Evidently,nondefectivenessimpliessuccess,butnot
conversely.Wegenerallyassumethatthereareonlytwowaysthatanactcanbe
successfullyperformedthoughstill bedefective. First,some ofthepreparatory
conditionsmightnotbethecaseintherealworldandyettheactmightstillbe
performed. The sincerity conditionsmight not obtain,in other words, the act
canbesuccessfullyperformedeventhoughitbeinsincere.
4.2 The Conditionsof Satisfaction
Theconditionsofsatisfactionofelementaryillocutionaryactsoftheformhf;pi
are afunction ofthe truth-conditionsoftheirpropositionalcontentand ofthe
direction oft oftheirillocutionary force[18,20,21].Generally,thespeakerex-
presses the proposition p with the aim of achieving a success of t between
language and the world from a certain direction. On the basis of the previ-
ous considerations on direction of t (see Section 2), we adopt the following
propositionabouttheconditionsofsatisfaction ofillocutionaryactsin general
semantics.
Proposition 2: An illocutionary act hassertive;pi with thewords-to-
worlddirection oftissatisedin asituationsof utteranceipholds
in s. Formally, if we express the satisfaction with the words-to-world
directionoftbysatis wl
wd
,thenthispropositionbecomes:
satis wl
wd
(comm(S;H;hassertive;pi);s)p[s]^p[s
u
] (9)
withs = do(says:to(S;H;hassertive;pi);s
u ) and s
u
is thesituation of
utterance.
Thus,thesuccessoftbetweenwordsandthingsisachievedbythefactthat
theexpressedpropositionalcontentmatchesastateofaairsexistingingeneral,
independentlyintheworld.
Inthecaseoftheworld-to-wordsorthedoubledirectionoft,theconditions
ofsuccess ofcommissivesanddirectivesarepartoftheirconditionsofsatisfac-
tion. Indeed, unlike assertive utterances, commissive and directive utterances
haveconditionsofsatisfactionsthatarenotindependentoftheseutterances.An
assertionis\true"i itspropositional contentcorrespondstoan existingstate
of aairsnomatter howit gotinto existence.On theother hand,apromise is
keptoranorderisobeyedonlyifthespeakerorhearercarriesoutintheworld
afuture course ofactionbecauseof thepromiseorthe order.Similarly,adec-
larationis satisedonly ifthe speakermakesitspropositionalcontent trueby
sayingthatitistrueintheperformanceofthatdeclaration.Now,wecanstate
oftissatised(satis)insomesituationsipholdsinsbecauseofthe
performanceofthisillocutionaryact.
Assincerityconditionsofhcommissive;piandhdirective;piareint(S;p)and
int(H;p)respectively,andthat\intention"hasbeendenedasa\commitment
+having-plan"(seeDenition6)wecanrelatetheperformanceofcommissives
and directivesto theexecutionof plansby thespeakerS andhearer H. Todo
this,weassumethatinhas:plan(i;;p)representsthesetofacts[
1
;:::;
n ].
In these conditions, do(
n
;do(
n 1
;:::;do(
1
;s)))::: ) is a situation denoting
the world historyconsisting of thesequence of actions [
1
;:::;
n
]. Inlight of
theseconsiderations,weconsiderrstlythesatisfactionofhcommissive;pi)and
hdirective;pi) bystatingthetwofollowingpropositions:
Proposition3:Anillocutionaryacthcommissive;piwiththeworld-to-
wordsdirectionoftissatisedinsomesituationsipholdsinsbecause
oftheperformanceofthisillocutionaryact.Formally,ifweexpress the
satisfactionwiththeworld-to-wordsdirectionoftbysatis wd
wl
,thenthis
propositionis:
satis wd
wl
(comm(S;H;hcommissive;pi);s)
9s 0
;s 00
(ss 0
s 00
)Poss(
i
;s 0
):::Poss(
n
;s 0
)^
success(comm(S;H;hcommissive;pi);s 00
)
p[do(
n
;do(
n 1
;:::;do(
1
;s 0
)))::: )
(10)
Proposition 4: An illocutionary act hdirective;pi with the world-to-
words direction of t is satised in some situation s i p holds in s
becauseof theperformance ofthisillocutionaryact.As thesatisfaction
withtheworld-to-wordsdirectionisexpressedbysatis wd
wl
,wecanstate:
satis wd
wl
(comm(S;H;hdirective;pi);s)
9s 0
;s 00
(ss 0
s 00
)Poss(
i
;s 0
):::Poss(
n
;s 0
)^
success(comm(S;H;hdirective;pi);s 00
)
p[do(
n
;do(
n 1
;:::;do(
1
;s 0
)))::: )
(11)
Notice that in the case of hcommissive;pi the agent which is in charge
of the execution of the set of acts [
1
;:::;
n
] is the speaker S, whereas in
the caseof hdirective;pi itis the hearerH. Finally, thesituation srepresents
do(
n
;do(
n 1
;:::;do(
1
;s 0
)))::: )ands 0
isthesituation inwhichallprecondi-
tionsof[
1
;:::;
n
]hold,nally,s 00
isthesituationwherecomm(S;H;hdirective;pi)
issuccessfullyperformed.
Asstatedpreviously,ahdeclarative;piissatisedonlyifthespeakerSmakes
itspropositionalcontenttruebysayingthatitistrueintheperformanceofthat
direction of t is satised in some situation s i p holds in s because
oftheperformanceofthis illocutionaryact.If thesatisfactionwith the
double direction of t is expressed by satis
dble
, then this proposition
becomes:
satis
dble
(comm(S;H;hdeclarative;pi);s)
success(comm(S;H;hdeclarative;pi);s)
p[do(says:to(S;H;hdeclarative;pi);s
u )]
(12)
Finally,inthecaseoftheemptyornulldirectionoft,thereisnoquestionof
successorfailure oft,andthepropositionalcontentis ingeneralpresupposed
to be true. Such are the expressives that have null or empty direction of t.
Theirpoint isto expressamental attitude ofthespeakerS aboutthestateof
aairs represented by p. Formally, we represent this by m(S;p) where m is a
psychologicalmodewhichdeterminesaparticulardirectionoftbetweenmind
and theworld,and pisthe propositionalcontentwhich representsthestateof
aairstowhich theyaredirected.Aswesee,in thecaseoftheemptydirection
oft,itisnottorepresentthatstateofaairsasactualortotrytogetittobe
actualintheworld.
Proposition 6: An illocutionary act hexpressive;pi with the empty
direction oft is satisedin somesituation s i m(S;p) holds in s be-
causeoftheperformanceofthisillocutionaryact.Ifthesatisfactionwith
theempty direction of t is expressed bysatis
;
, then this proposition
becomes:
satis
;
(comm(S;H;hexpressive;pi);s)
success(comm(S;H;hexpressive;pi);s)
m(S;p)[do(says:to(S;H;hexpressive;pi);s
u )]
(13)
Finally,itisimportanttonotethatthe setofillocutionaryforces ofpossible
utterancesisrecursive.Consequently,there areveprimitiveillocutionaryacts
withanillocutionaryact,nospecialmodeofachievementofthatpoint,aneutral
degreeofstrengthandonlythepropositionalcontent,andthepreparatoryand
sincerityconditionswhicharedeterminedbytheirpoint.Theseprimitiveforces
are : kassertk for the assertives, kcommitk for the commissives, kdirectk for
thedirectives,kdeclarekforthedeclarativesandnally,theprimitiveexpressive
illocutionary force which is realized syntactically in the type of exclamatory
sentences(thereisnoillocutionaryverborperformativein Englishthat names
the primitiveexpressive).kkis thefunction that assignsto each illocutionary
verb the force or type of speech act that it names. Such a function can be
tainedfromtheprimitiveillocutionaryforcesbyanitenumberofappli-
cationofthefollowingoperations:i)addingpropositionalcontentcondi-
tions,ii)addingpreparatoryconditions,iii)addingsincerityconditions,
iv) restricting the mode of achievement and nally, v) increasing the
degreeofstrength.
TheseoperationsareBooleanoperations(see[21]fordetails).Herearesome
examples of derived illocutionary forces. The illocutionary force kpromisek is
obtained from the primitive commissive force kcommitk by imposing a spe-
cial mode ofachievementofthe commissivepoint involvingtheundertakingof
anobligation. kpledgekisobtainedfrom kcommitkbyincreasing thedegreeof
strengthofthesincerityconditions,etc.
5 Applications of our Approach
5.1 Contributions to the Analysis and Interpretation of Speech
Acts
Our semantics,expressed into a tractable language(the situation calculus), is
usefulforseveralreasons.First,itenablesustoanalyzeillocutionaryforcemark-
ers and to interpret agreat number of speech acts. Second, it also enables us
to makeareasoneddictionaryofillocutionaryverbs ofactualnaturallanguage
by wayof a systematicbreakdown of lexicalizedforces into theircomponents.
Third,ourtheory isalso usefulto describe thevarioussorts ofentailmentand
relativeinconsistency that canexist betweenactualsentencesexpressed in the
same contexts with relatedconditions of success and satisfaction. Such entail-
ments and inconsistencies might be important in the context of Agent Com-
municationLanguage(ACL). Forexample,asentenceS
1
suchas\I askyouif
youare busy" illocutionarily entails the sentence S
2
\Are you busy?". Thus a
speakercould notperform S
1
withoutalsoperformingS
2
inthe samecontext.
Similarly,thetwosentences\Howniceofyoutonishthejobj
1
"and\Iregret
thatyouhavedonej
1
"areillocutionaryincompatiblebecausetheyexpressinthe
samecontextsspeechactsthat arenotsimultaneouslyperformable.Theformal
model presented here canalso generalize the traditional truth conditional no-
tionsofentailmentandconsistencyandapplythemtonon-declarativesentences.
Forexample,theimperativesentenceS
3
\Please,touch mebyemailtomorrow
morning!"canbesaidtoentailtruth-conditionallythesentenceS
4
\youareable
totouchmebyemailtomorrowmorning".Thus,S
3
expressesinagivencontext
arequestwhich cannotbesatisedunlesstheassertionexpressed byS
4 in the
samecontextistrue.Similarly,theimperativesentenceS
5
\emailpieceofinfor-
mationP
1
... anddo notemail P
1
!"is truth-conditionally inconsistent sinceit
expressesinallcontextsadirectivewhichisnotsatisable.
Thus, theapproach presented here canbeused to formally analyze illocu-
tionaryforces.Suchananalysisisusefultoformalizehumaninteractionsaswell
KQML performatives
The semantics developed by authors of KQML turns around the well known
approach: speakersand hearers haveonly to recognize each other's intentions.
This approach, which wasinitiated by[1]considers in fact that theonlykinds
of things that are intrinsically, as opposed to derivatively, meaningful are not
linguistic acts like the act of asserting that something is the case, the act of
requestingsomeonetomakesomethingthecase,ortheactofpromisingtomake
somethingthecase,butrathermentalstateslikethestateofbelieving,desiringor
intending.Withasuchapproach,someaspectsofthesuccessarenotconsidered
(as for instance the \degree of strength" and some facets of the preparatory
conditions)andcannotderivefromthementalstatesoftheinterveningparties.
Inourpointofview,thedegreeofstrengthandthepreparatoryconditionsare
veryimportantfortheinteractions amongagentsasexplainedin Section2.To
remedytothis,wegivehereoursemanticsforsomeperformativesusedinKQML
[8]:achieve(S;H;p), andtell(S;H;p).
Firstly, achieve(S;H;p) performative should be analyzed in our framework
asakrequestkaddressedbyS toH inordertoachievep.Noticethatrequestis
adirectiveillocutionaryactthat allowsforthepossibilityof refusalandconse-
quently,itcanbegrantedorrefusedbyH.Intheseconditions,krequestkdiers
fromtheprimitivedirectivekdirectk(seeprevioussection)onlybythefactthat
the mode of achievementallows H the possibility of refusing to carryout the
futurecourseofactionrepresentedbyp.Inourframework,thiscanbeexpressed
by:
success(comm(S;H;krequestk(p));s)
success(comm(S;H;kdirectk(p));s)^8s 0
(s 0
s 0
):oblig(H;S;p)[s 0
]
Theconditionofsatisfactionof comm(S;H;krequestk(p)) is determinedby
Proposition5.
Secondly, tell(S;H;p) performativeshould beanalyzed hereasan kassertk
thatp.Withoursemantics,allotherassertivesofKQML(untell,sorryanderror)
shouldbeanalyzedinourframeworkaccordingtotheProposition7.Precisely,
these assertivesare obtainedfrom theprimitiveillocutionaryforce kassertkby
a nite number of application of the following operations: i) adding proposi-
tionalcontentconditions,ii)addingpreparatoryconditions,iii)addingsincerity
conditions, iv) restricting the mode of achievement and nally, v) increasing
the degree of strength. In fact, all KQML performatives should be expressed
in ourframeworkusing recurrence.Inthisway,KQML\performatives"canbe
extended since developers have some guidance on how to formulate new per-
formatives.Thus in thecase ofassertivesforinstance, developershaveonly to
develop performatives which may dier from one another in respects such as
their mode of achievement (the dierence, for example, between arguing and
testifyingthatp);theirdegreeofstrength(the dierence,forexample,between
insistingthat pandconjecturingthat p),theirpropositionalcontentconditions
(the dierence,forexample,betweenapredictionandareport),theirprepara-
sertingandcomplaining).
6 Conclusion
Themodelofspeechactspresentedinthispaperrepresentsasystematic,unied
accountof both the truth{andthe success{conditional aspects. In this model,
meaningandusearelogicallyrelatedandlinguisticcompetenceisnotdissociated
from performance.Onthecontrary,linguisticcompetenceisconstructedasthe
speaker'sability to understand which illocutionary acts can be performed by
literalutterancesinthevariouspossiblecontextsofuseofthislanguage.
Wehaveproposedanadequateformalism(the situationcalculus) forrepre-
sentingthismodelinthecontextofagentcommunicationlanguage.Finally,we
haveexplainedhowtheresultingmodelallowsusto(1) contributeto theanal-
ysisand interpretation ofspeech acts; (2)contribute tothe semanticsof agent
communicationlanguageasKQML.
This work is only the beginning and we plan to extend it, in particulier
to conversationstaking into account social interactions of autonomous agents,
and specially the private and global views on communicationas developed by
Dignum[6].
Acknowledgments:
ThisresearchwassupportedbytheNaturalSciencesandEngineeringResearch
Council ofCanada(NSERC),bytheSocialSciencesand Humanities Research
Council(SSHRC)ofCanada,bytheFondspourlaFormationdesChercheurset
l'aidealaRecherche(FCAR)duQuebecandinpart,bytheGermanResearch
CentreforAI(DFKI).
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