better, alternatives.
CQAO-01. Is there an alternative plan to promote the same valuevP L?
An agent ag has reason to believe there is an alternative plan to promote the same values iff in its local plans exists a planP Lxthat promotes the same set of valuesvP L. Example: Let’s assume agentN GO presents plan P LA Is there an alternative plan to P LA that promotes the same values? Agent P KF has reason to present plan P LC on the basis that promotes the same set of values (see Tables A.21 and A.22).
CQAO-02. Is there an alternative plan to realise the same new circumstances?
An agent ag has reason to believe there is an alternative plan to reach the same new circumstances iff in its local plans exists a planP Lx that realises the same new circumstances.
Example: Agent N GO There is an alternative plan to P LG being plan P LG1 that achieves the same new circumstances in less time.
CQAO-03. Is there an alternative plan to realise the same goal?
An agent ag has reason to believe there is an alternative plan to realise the same goal iff in its local plans exists a plan P Lx that realise the same goal.
Example: There is an alternative plan to P LG being plan P LD that achieves the goal villagers in zone 14.
3.6
Chapter Summary
This chapter presents an argumentation scheme for plan proposals together with critical questions that challenge the scheme at several levels. multi-agent planning is known to be a highly complex and detailed problem due to the need to represent and reason about a large number of elements. I described an argumentation-based approach that captures how multi-agent plans can be criticized and therefore justified using a large number of questions.
The practical reasoning scheme in [9] was extended with durative actions to present argumentation scheme for plan proposals. Although [9] may reason about plans as single monolithic actions, the extension presented allow us to consider thoroughly the plans taking into account their components. To specify multi-agent plans I consider individual temporal plans that have temporal constraints between them. An analysis action combinations also has been presented to in order to tease out the ways in which actions can concur in a multi-agent plan.
The set of critical questions is categorized in seven layers that focus on different aspects of the proposal going from the lowest level which refers to actions, to the highest level which inquires about alternative options. A common remark when using critical questions as a way to challenge argumentation schemes is how to be sure that the full set of possible questions has been considered. Whilst I leave open the possibility for further questions to be added to the categories, I have generated the list from a systematic analysis of the various elements of the argumentation scheme and hence believe that it can be taken as complete for the current purposes. The critical questions address each element of a proposed plan and so they are comprehensive with respect to the representation I have chosen for plan proposals. Furthermore, the reason to classify questions suggests that another layer could be added to the list or more questions could be added to a specific layer to complete the analysis for more specific purposes.
The contribution of this chapter in relation to the thesis is that it enables a plan proposal to be considered automatically by software agents engaged in a dialogue over plans. Another important contribution is that I have articulated a novel list of critical questions related to an argumentation scheme for plan proposals that includes temporal aspects. The argumentation scheme presented is as part of the dialogue game protocol in the next chapter in which agents can propose plans and critique the proposals through specific locutions using the questions presented in this chapter.
A Planning Dialogue Game
Protocol
In the previous chapter I introduced an argumentation scheme for plan proposals where plans are conceptualized as scheduled combinations of actions and an analysis of the critical questions that match the scheme at several levels. In this chapter, I present thePlanning Dialogue GameProtocol (PDGP), that allows agents to evaluate plans using the critical questions from the previous chapter. Agents can use then the protocol to engage in a dialogue to propose, critique and modify plans using elements from persuasion and deliberation dialogue protocols. The protocol allows the proposal to be discussed at the action level using the same argumentation scheme and critical questions, so enabling the proposal to be discussed at all of the levels identified.
I explained in Section 2.4.3 the importance of having a mixed-type protocol to enable a comprehensive argumentative dialogue about plans. The PDGP protocol is based on persuasion and deliberation protocols deigned for agent communication but could be seen as a “Discovery Dialogue” (as in [107]) where the goal is to choose the best plan given that agents need to find an explanation or justification of plans in terms of the actions that comprise them.
The remainder of this chapter is structured as follows: Section 4.1 presents the underlying theory of the PDGP protocol that enables the critique and modification of plans. The Section is structured as follows: Subsection 4.1.1 presents the roles agents can take when using the protocol and thedialogue stages that comprise the protocol and subsection 4.1.2 presents the main rules of the protocol. Subsection 4.1.3 presents the syntax of the protocol. Subsection 4.1.4 presents thesemantics of the protocol in terms of preconditions and postconditions. Section 4.2 presents a protocol evaluation based on the desiderata for dialogue argumentation protocols in [112]. Section 4.3 presents two example dialogues using the protocol based on the example from Appendix A. Finally, Section 4.4 presents the chapter conclusion.
4.1
PDGP: A Planning Dialogue Game Protocol
In argumentation-based dialogues information is shared using formalisms that specify the rules to exchange locutions between agents. Agent interaction based on dialectical argumentation consists of agents presenting arguments for and against a claim that conforms to protocol rules. Specifically, an agent interaction protocol provides valid locutions and the semantics of these locutions so that agents can present arguments to enable communication (a discussion on the transition from agent communication languages to protocol-based communication is presented in Section 2.2.3).
ThePDGP protocol embodies a theory of justification for plan proposals that allows agents to agree on a plan to execute following the argumentation theory from Walton [186, 187, 190]. ThePDGP protocol takes elements from on three known argumentation- based protocols (all of which were discussed in Section 2.2.3.5):
• the Fatio protocol [110], a protocol that permits agents to make, question, chal- lenge and justify assertions, using an explicit argumentation theory,
• thePARMAprotocol [10], a dialogue game protocol for action proposals and the
• the Command Dialogue Protocol CDP [13], a dialogue protocol for command dialogues.
In [58, 132], is presented a discussion on the the importance of having agents able to handle mixed types of dialogues in cooperative scenarios. When agents need to present proposals for plans, evaluate them and possibly modify them, several types of dialogue could be used. Planning scenarios like the one presented in Appendix A can be very complex and can require in principle several types of dialogue. Agents may try to persuade each other to agree on a plan or action, or may deliberate on the best action given a point where there are only invalid plans. To create a plan information-seeking dialogues would also be required in order to retrieve more information from other agents to create the plan or agents couldnegotiate over the plan resources.
The PDGP takes a persuasion and deliberation approach. The purpose of creating a protocol that enables first persuasion and possibly deliberation is to allow agents to persuade each other with their already created plans and modify plans if necessary avoiding a complete re-planning process. When agents identify the action that causes a conflict agents can re-plan or deliberate over the next course of action from a point in time. In Persuasion Dialogues participants ask for and provide reasons for their claims; in consequence, the information agreed by the participants changes during a dialogue and the goal of a persuasion dialogue is to resolve a conflict of opinion1 [142]. The protocol presented thus aims to fulfil a specific need arises from the issues when critiquing plans and it is by no means comprehensive for all the possible types of dialogue necessary for a comprehensive dialogue about plans in terms of creation and execution (see Section
1
Walton’sThe New Dialectic[187], suggests persuasion is only about beliefs but in later work Walton talks about persuasion also being over actions [12].
2.4.3 for a list of dialogue variations about plans). The next subsections presents in detail teh characteristics of the protocol.