A Comprehensive Theory of Meaning forCommunication Acts in Multi-Agent Systems

Christian Lemaître1 Amal El Fallah-Seghrouchni LANIA, Mexico LIPN, France

cll@xalapa.lania.mx elfallah@lipn.univ-paris13.fr

Abstract

Based on the Communicative Action Theory proposedby the German Philosopher J. Habermas, we propose anovel framework and formalism that goes beyond theclassical speech act theory and its intentionalisticinterpretations [1,3,5,6]. We introduce a comprehensivetheory of meaning for communication acts assuming thatthe content of natural language utterances can beclassified in three different domains of discourse, each onewith a different type of semantic validation: the domain ofobjective facts, the internal or subjective domain of thesender, and the social relational domain of the sender andthe receiver. We introduce also a crucial shift in the agentinteraction approach, focusing on the conversation controlissues and the key role played by the receiver. We claimthese two new approaches of mutiagent interactions willallow to control and manage the complex interactionsamong agents in open real World applications.

1. Introduction

We present a new agent interaction formalism based onHabermas / Bühler communication acts theory [4].

Interaction among agents implies complex patterns ofspeech act interchanges. If these message interchanges areto be fruitful, interlocutors must be able to decide if eachmessage they receive is or not acceptable. If theythink a message is not valid they must reject it, eventually,with a criticism or a reason why they did not accept it.____________________

1Partial research support has being provided by 25509-A grant fromMexico' s National Council of Science and Technology, CONACYT

If the receiver R accepts the message he can then interpretit and act accordingly.

We claim that what it is needed at present stage ofMAS real applications in open domains as electronicbusiness, is a formal theory helping to control meaningfulspeech act interchanges among agents. We need a theory ofmeaning based on the listener, the one who receives thespeech act, and the one who can accept or reject it as validor invalid. With this shift in our focus, looking at thereceiver instead of the sender, we can assure real control ofagent interactions. Following Habermas, we claim that thecontent of natural language utterances are about three maintopics: the domain or world of objective facts (W), theinternal or subjective domain of the sender (I), and thesocial relational domain of sender and receiver (So). Eachdomain has different validity criteria. We assume thefollowing validity criteria: For W we assume the classicalTrue/False semantics; for I we assume aTruthful/Untruthful semantics; and for So we assume aRight/Not-right semantics. Any utterance might becriticized from the three points of view, although, there isalways a more natural criticism for each type of speech act.For instance an “inform” utterance classified as W shouldbe expected to be criticized from a true/false semantics, butalways R can consider S as a liar, and reject his utterancein the I perspective.

2.Validity claim procedures

TRUE / FALSE VALIDITY. This procedure impliesthat a third person can verify the truth or untruthfulness ofthe fact. R can accept an assertoric sentence if he knowswhat kind of reasons S would have to cite in order toconvince him his assertion is true. If R has some doubtsabout the reasons S may have, he can reject the utterance orask for more arguments about the validity of the assertion.

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TRUTHFUL / UNTRUTHFUL VALIDITY. Thisprocedure implies that R can decide if he trusts or not S,according what S has said and also on his past experiencewith S, i.e., on his truthfulness model of S. If thetruthfulness of S is not good enough, R may: 1) refuse S´sutterance because he does not trust him; or 2) try to ensuresome commitment of S that gives him some confidence thatthe intention expressed by S will become a socialcommitment. In this case, R will shift the domain ofdiscourse from I to So.

If R trusts on S´ Intention and S does not keep his word,R can: 1) automatically modify the truthfulness parameterin his S´s truthfulness model; or 2) initiate a conversationasking S why he did not keep his word. If S gives R goodreasons, R will store in his S model that the failure had agood explanation.

RIGHT / NOT RIGHT VALIDITY: In So-domain Rmust analyze if S is violating (or not) a social norm of anysort or a social commitment between S and himself. Thisprocedure implies the existence of shared norms among Sand R, as well as the existence of a third agent who caneventually enforce the norm compliance. We include,social commitment in the sense of Castelfranchi and co-workers [2].

3. The S model

A main tool for utterance validity analysis is the modelthe receiver must have of the sender. In fact agent R needsto have a three-fold model of its interlocutor along thethree discourse domains (W), (I) and (So). The W-modelshould have the facts he knows about S, name, e-address,etc. as well as information about S’ ability to have goodquality beliefs, i.e. beliefs that are true. The I-model shouldstore intentions and other subjective feelings of S and hisdegree of truthfulness. The So-model should storeinformation about S’s social roles, its power, permits, aswell as its social credibility.

4. Utterance acceptance procedure

We represent a communication act as a 3-tuple<F,D,p>, where F is the illocutionary force of thecommunicative act, D = {W, I, So} is the discoursedomain, and p is the proposition content of the utterance.The core decision process of the receiver agent R, to acceptor reject an utterance sent by agent S is as follows:Identification phase, Syntactical verification, and Validityanalysis.

IDENTIFICATION PHASE: The first stepcorresponds to the recognition of the sender S, theperformative F, and the domain D of the utterance.

SYNTACTICAL VERIFICATION: As second step Rperforms a syntactical verification based on the structure ofthe communication act <F, D, p> and the relationshipexisting between R and S. This analysis is made in twofaces. The first one, the F/D adequacy, is to verify that theperformative F can be uttered within the domain D. Thesecond one is the Sender/ <F,D> adequacy when R verifiesif the S/R relationship allows S to use such a performative.

VALIDITY ANALYSIS: Once the syntactic analysishas been finished, R must carry out a semantic analysis ofthe propositional content. The aim of this phase is tow-fold: 1) to decide if R will accept or reject S utterance, and2) to interpret the utterance and execute the entailedactions. The validity analysis depends on the discoursedomain D. As mentioned before, three types of validityclaims are considered: the True/False for W-domain, TheTruthful/Untruthful for I-domain, and the Right/No-rightfor So-domain. The purpose here is to go beyond thesyntactic analysis of the previous stage, in order to analyzethe meaning of propositional content "p", verifying theconsistency of the content with the illocutionary force F,the discourse domain D, and the sender model. Thesyntactic verification has analyzed the formal consistencyof S, F and D. This analysis is not enough in most of thecases. Suppose, that S is the boss of R, and thus he has theright to give him some orders, but he does not have theright to order R to do some action not allowed byenterprise regulations. The syntactic analysis will acceptthe communication act <Order,So,p>. It is thus necessaryto do a semantic interpretation of "p" to be sure if the orderis valid or not. If the communication act is valid, R shouldaccept it and initiate the procedure entailed by it.

References

[1] J. L. Austin. How to Do Things with Words. Clarendon Pres.Oxford, 1962.

[2] C. Castelfranchi. Modeling social action for AI agents. InArtificial Intelligence 103. Elsevier, 1998

[3] P. R. Cohen and C.R. Perrault. Elements of a Plan-Basedtheory of Speech Acts. In Cognitive Science 3:177-212,1979.

[4] J.Habermas. Postmetaphysical Thinking. The MIT Pres.Cambridge, 1996.

[5] A. S. Rao and M. P. Georgeff. Modeling rational agentswithin a BDI-architecture. In Proceedings of the InternationalConference on Principles of Knowledge Representation andReasoning. 1991.

[6] J.Searle. Speech Acts. Cambridge University Press. 1969

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