Conversations are sequences of messages exchanged between two or more agents. While fixed protocols are too rigid to properly deal with teamwork dynamics, offering complete freedom in communication would be too much for resource-bounded software agents at the
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current state of communication technology. Therefore a form in between the two extremes, namely dialogue theory, has been en vogue recently (Cogan et al., 2005; McBurney et
al., 2002; Parsons et al. 1998, 2003).
Dialogue theory has been influenced by parallel developments in logic and philosophy in the 1960s and 1970s. Among other researchers, Hintikka (1973), Kambartel (1979), Lorenzen (1961) and Krabbe (2001) developed the idea that semantics of classical and intuitionistic logics could be alternatively formalized in terms of games among two play- ers, instead of the usual Tarski and Kripke semantics. This more dynamical view of semantics has inspired many developments in philosophy, logic and theoretical computer science, for example the invention of dynamic epistemic logics (Baltag et al., 2003, 2008; Benthem, 2001; Ditmarsch et al., 2007).
One of the inventors of dialogue logics, Erik Krabbe, joined forces with Douglas Walton to create a freer version, dialogue theory, that is geared more to modeling real- life dialogues than to the semantics of classical or intuitionistic logic. They classified several dialogues: persuasion, negotiation, inquiry, deliberation, information seeking and
eristics, with a special focus on persuasion. As we consider only cooperative teams,
eristics, that is verbal fighting between agents, has been left out in the sequel. For each type of dialogue, Walton and Krabbe (1995) formulate an initial situation, a primary goal and
a set of rules. These constitute a normative model which is not a record of real dialogues,
but represents the ideal way cooperative agents participate in the dialogue in question. In the course of real-life communication, often a shift from one type of dialogue to another occurs. A special kind of shift, called embedding, takes place when the second dialogue is functionally related to the first one and improves its results. For example, persuasion about a certain statement may need an information-seeking phase.
Dialogue theory structures conversations by means of a number of dialogue rules. These rules limit the number of possible responses at each point, while not fixing the sequence of messages. The agents speak in turn, for example asking questions and giving replies and take into account, at each turn, what has occurred previously in the dialogue. The score of the dialogue is kept by each agent as an attitude store, to which propositions may be added or retracted during the dialogue in an orderly way. These propositions classically represent informational attitudes like individual beliefs or common beliefs. As a novelty in MAS, they may also represent motivational attitudes like individual goals, individual intentions, collective intentions, social commitments and collective commitments.
Below we shortly explain dialogue theory and briefly describe the speech act theory used to implement the effects of utterances in dialogues between computational agents.
8.2.1 Persuasion
A persuasion dialogue arises from a conflict of opinions. It may be that one agent believes
ϕ while some others either believe a contrary proposition ψi (whereϕ∧ ψiis inconsistent) or just have doubt aboutϕ. The goal of a persuasion is to resolve the conflict by verbal
means, so as to ensure a stable agreement. In the multi-agent setting, the end result would be a common belief C-BELG(χ ), where χ may be the ϕ or one of the ψi, or yet another conclusion resulting from persuasion. Clearly, belief revision takes place here.
Initially, all agents have attitude stores consisting of theses and concessions. Here, the theses are assertions they are prepared to defend (likeϕ for the first agent above), while
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concessions are propositions that are taken for granted for the sake of argument. Walton and Krabbe provide many rules governing effects of updates, revisions and retractions on the attitude stores during persuasion.
In the MAS setting, a persuasion with respect to motivational attitudes, not found in Walton and Krabbe, has to be introduced. This new kind of persuasion arises from a conflict of intentions, where one agent intends to achieveϕ, while others have a conflicting
intention to achieve ψi (where ϕ and ψi are inconsistent) or simply lack any positive motivational attitude with respect to ϕ. The main goal of persuasion with respects to
intentions is to resolve this conflict in a way resulting in a stable collective intention.
8.2.2 Negotiation
The initial situation of negotiation is a conflict of interests, together with a need for cooperation. The main goal is to make a deal. Thus, the selling and buying of goods and services often described in the MAS literature is only one of the many contexts where negotiation takes place in multi-agent systems. Negotiation and persuasion are often not distinguished adequately. One has to keep in mind that negotiation is not meant to con- vince the others of one’s viewpoint, as happens in persuasion, but to make a deal leading to a mutually beneficial agreement. There is a wide literature on negotiation in multi-agent systems, covering an area as wide as exchange of services, sale of products and develop- ment of treaties among nations (Kraus, 2001; Lin and Kraus, 2008; Sycara, 1990). Formal techniques for negotiation have recently received a lot of attention, from Rosenschein and Zlotkin’s Rules of Encounter (Rosenschein and Zlotkin, 1994), through information-based negotiation by among others Sierra and Debenham (2007), to game-theoretic approaches by among others Ramchurn et al. (2007). We do not go into details here.
In general, Walton and Krabbe (1995) do not allow us to embed negotiation into persuasion, assuming that a proposed statement should be backed by arguments, not offers. When an agent in the course of persuading another agent begins to negotiate, it may be accused of escaping from the burden of proof. Walton and Krabbe call such an illicit embedding of negotiation into persuasion the ‘fallacy of bargaining’. On the other hand, persuasion may be fruitfully embedded in negotiation. For example, when setting up the agenda, or in a negotiation about the sale of a house, an embedded persuasion about the market value of similar houses in the neighborhood typically helps clinch the deal.
The rules governing negotiation include severe restrictions on retracting concessions, which are represented mostly as courses of action. In general, when an agent has conceded its willingness to execute some action (for example to sell a product for a certain price) it may not generally retract this concession.
8.2.3 Inquiry
Inquiry starts when some agents are ignorant about the solution to some question or open problem. The main goal is the growth of knowledge, leading to agreement about the conclusive answer of the question. This goal may be attained in many different ways, including an incremental process of argument which builds on established facts in drawing conclusions beyond a reasonable doubt. Both information retrieval and reasoning may be intensively used in this process. The end result of inquiry has a collective flavor and is
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as strong as C-BELG(ϕ) or even C-KNOWG(ϕ) in some contexts. If one agent reaches an intermediate or final conclusion earlier than others, it may need to persuade them. Therefore, a persuasion dialogue is allowed in inquiry. Conversely, if an open problem appears during persuasion, an inquiry may be embedded to resolve it.
8.2.4 Deliberation
Deliberation as a dialogue is similar to inquiry, but different from both persuasion and negotiation as it starts from an open problem, rather than from a conflict of opinions. Deliberation starts from a need for action performance and is concerned with the future. It aims to reach a decision on how to act in the short term. The kind of reasoning that is central to deliberation and in general to teamwork in multi-agent systems, is practical
reasoning: goal-directed, knowledge-based reasoning where an agent considers different
means of achieving a goal. A typical example of practical reasoning is a means-end analysis linking a particular goal or intention with a, possibly complex, action.
8.2.5 Information Seeking
Information seeking occurs when an agent lacks knowledge on a certain subject or propo- sition and it seeks this information from others. The end result is a new individual belief BEL(a, ϕ) of the interested agent a. In contrast to inquiry, the attainment of proof is
not essential in information seeking. Apart from collective aspects of inquiry, this dis- tinguishes the two potentially similar dialogues. Information seeking typically occurs in expert consultation, when the questioner has no direct access to information.