Desiderata for agent argumentation protocols.
Designers of agent communications protocols are increasingly using formal dialogue games, adopted from argumentation theory, as the basis for structured agent interactions. We propose a set of desiderata for such protocols, drawing on recent research in agent interaction, on recent criteria for assessment of automated auction mechanisms and on elements of argumentation theory and political theory. We then assess several recent dialogue game protocols against our desiderata, revealing that each protocol has serious weaknesses. For comparison, we also assess the FIPA Agent Communications Language (ACL), thereby showing FIPA ACL to have limited applicability to dialogues not involving purchase negotiations. We conclude with a suggested checklist for designers of dialogue game protocols for agent interactions.
INTRODUCTION
Formal dialogue games are games in which two or more participants
"move" by uttering locutions, according to certain pre-defined
rules. They have been studied by philosophers since the time of
Aristotle, most recently for the contextual modeling of fallacious
reasoning [14, 23] and as a proof-theoretic semantics for intuitionistic
and classical logic [22]. Outside philosophy, dialogue games
have been used in computational linguistics, for natural language
explanation and generation, and in artificial intelligence (AI), for
automated software design and the modeling of legal reasoning,
e.g., [4]. In recent years, they have found application as the basis
for communications protocols between autonomous software
agents, including for agents engaged in: negotiation dialogues, in
which participating agents seek to agree a division of some scarce
resource [3, 17, 27, 30]; persuasion dialogues, where one agent
seeks to persuade another to endorse some claim [2, 7, 8]; information-seeking
dialogues, where one agent seeks the answer to some
question from another [17]; inquiry dialogues, where several agents
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jointly seek the answer to some question [24]; and deliberation di-
alogues, where participants seek to jointly agree a course of action
in some situation [16]. 1
Why have agent protocol designers turned to dialogue games
from argumentation theory? It is reasonable to assume that a rational
agent would only change its beliefs or its preferences after
receiving new information, i.e., not on the basis of whim or malice,
say. For an agent to acquire new information in a dialogue, it needs
a way to probe or challenge the statements of other agents; thus,
locutions which enable utterances to be questioned or contested are
required, along with locutions which enable appropriate responses
to these. In order that this process takes place in an orderly and
efficient fashion, we require rules which govern what can and cannot
and must be said in a dialogue, and when. Dialogue games
provide a framework for the design of such structured discourses,
drawing on specific theories of argumentation. We would expect
that the greater the amount of relevant information passed between
participants, the greater is the likelihood of successful resolution
of the dialogue; increased likelihood of resolution is therefore the
expected payoff of these games when compared with more parsimonious
interaction protocols, such as auctions.
However, despite this recent interest in dialogue-game protocols
for multi-agent systems, we know of no discussion of appropriate
design principles. As these protocols proliferate, designers
and users will require means to assess protocols and to compare
one with another. In this paper, we therefore propose the first
list of desiderata to govern the design and assessment of dialogue
game protocols. To do this, we have drawn upon: the criteria recently
proposed for assessment of automated auction and negotiation
mechanisms in, e.g., [31]; theories of deliberative decision-making
from argumentation theory [1, 15] and political theory [6,
10, 12]; and recent studies of agent communications languages and
interaction protocols [13, 20, 33, 36]. We believe our list of desiderata
will be an initial step towards the development of formal design
and assessment criteria for agent argumentation protocols.
2. PROPOSED
We begin by assuming that agents engaged in dialogues are au-
tonomous, willing and free participants, able to enter and withdraw
from dialogues as and when they see fit. Within each dialogue, they
remain autonomous, and are not compelled to accept or reject any
proposition. These assumptions have implications for some of the
desiderata, as explained below. We also assume that the specification
of a dialogue game protocol consists of: (a) a set of topics of
discussion (which may be represented in some logical language);
This typology of dialogues is from [35], and we use it throughout this paper; note
that negotiation and deliberation dialogues are defined more precisely than is usually
the case within AI.
(b) the syntax for a set of defined locutions concerning these top-
ics; (c) a set of rules which govern the utterance of these locutions;
(d) a set of rules which establish what commitments, if any, participants
create by the utterance of each locution; and (e) a set of rules
governing the circumstances under which the dialogue terminates.
We refer to such a specification as a Dialectical System. 2 We now
list our desiderata with a brief explanation for
1. Stated Dialogue Purpose: A dialectical system should have
one or more publicly-stated purposes, and its locutions and
rules should facilitate the achievement of these. For exam-
ple, the stated purpose of a system for negotiation may be
an agreement on the division of a particular scarce resource;
negotiation over a different resource will result in a different
purpose. Likewise, a discussion about the same resource
which is not a negotiation over its division constitutes a different
purpose, e.g., it may be an information-seeking dia-
logue. The dialogue purposes need to be stated, so that all
participating agents are aware of them in advance of entering
the dialogue. Successful resolution of a dialogue will occur
when its stated purposes are achieved.
2. Diversity of Individual Purposes: A dialectical system should
permit participating agents to achieve their own individual
purposes consistent with the overall purpose of the dia-
logue. These individual purposes may conflict, as when parties
to a negotiation each seek to maximize their individual
utility in any outcome, or they may coincide, as when agents
collectively seek to answer some unknown question.
3. Inclusiveness: A dialectical system should not preclude participation
by any potential agent which is qualified and willing
to participate. Because agents are autonomous entities,
there is a sense in which all agents are deserving of equal
respect. As with human beings [6], agents affected by decisions
have a moral right to be included in deliberations
leading to those decisions. In addition, inclusion of affected
parties in decisions can improve the quality of the decision
outcomes [10].
4. Transparency: Participants to a dialogue should know the
rules and structure of the dialectical system prior to commencement
of the dialogue. In particular, any reference from
dialogues in a dialectical system to an external reality should
be explicitly stated, and known to the participants before
commencement, e.g., when commitments incurred inside a
purchase negotiation dialogue imply subsequent real-world
obligations to execute a particular commercial transaction.
5. Fairness: A dialectical system should either treat all participants
equally, or, if not, make explicit any asymmetries in
their treatment. For instance, it may be appropriate for participants
to play different roles in a dialogue, such as sellers,
buyers and auctioneers in a purchase transaction dialogue
[32]. Agents in these different roles may have different rights
and responsibilities, and these should be known to all.
6. Clarity of Argumentation Theory: A dialectical system
should conform, at least at the outset, to a stated theory of
argument, for example Hitchcock's Principles for Rational
Mutual Inquiry [15] or the persuasion dialogue rules of [9].
This model is presented in [25]. Note that there is no consensus among philosophers
over distinctions, if any, between the words "dialogical" and "dialectical" [5, p. 337].
The reason for this is so that all participants know, and adhere
to, their dialectical obligations, agree on rules of inference
and procedure, and have reasonable expectations of the
responses of others. For example, an agent should know in
advance of making an assertion that its statement may incur
obligations to defend it upon contestation by others; like-
wise, agents contesting an assertion should know if they are
entitled to receive a defence of it. The dialogue-game rules
which embody a theory of argumentation ensure that such
arguments are conducted in an orderly and efficient manner.
If dialogue participants wish to change the argumentation-
theoretic basis or the dialogical rules of the system in the
course of using it for a particular dialogue, being free agents,
they should be enabled to do so. 3
7. Separation of Syntax and Semantics: The syntax of a dialectical
system should be defined separately from its seman-
tics. There are two reasons for this. Firstly, this approach
enables the same protocol syntax to be used with multiple
semantics. Secondly, the problem of semantic verification
of an agent communications language is a thorny one [36],
since it will always be possible for a sufficiently-clever agent
to simulate insincerely any required internal state. Ensuring
that the protocol syntax is defined separately from its semantics
therefore enables the verification of conformity with protocol
syntax, even if the protocol semantics cannot be completely
verified. 4 The recent development of a social seman-
tics, where agents first express publicly their beliefs and intentions
relevant to an interaction, may be seen as an attempt
to extend the domain of verifiability [33].
8. Rule-Consistency: The locutions and rules of a dialogue
system should together be internally consistent; that is, they
should not lead to deadlocks (where no participant may utter
a legal locution), nor infinite cycles of repeated locutions.
9. Encouragement of Resolution: Resolution of each dialogue
(normal termination) should be facilitated, and not precluded,
by the locutions and rules of a dialectical system.
10. Discouragement of Disruption: Normally, the rules of a dialectical
system should discourage or preclude disruptive be-
haviour, such as uttering the same locution repeatedly. How-
ever, as Krabbe notes with regard to retraction [18], achieving
a balance between outlawing disruptive behaviour and
permitting freedom of expression is not necessarily straight-
forward, and will differ by application.
11. Enablement of Self-Transformation: A dialectical system
should permit participants to undergo self-transformation [12]
in the course of a dialogue; e.g., participants to a negotiation
should be able to change their preferences or their valuations
of utility as a result of information they receive from others in
the dialogue, or express degrees of belief in propositions. In
particular, participants should have the right to retract commitments
made earlier in the same dialogue, although not
necessarily always unconditionally. If the protocol does not
permit such transformation, then one agent would not be able
to persuade another to change its beliefs or to adopt a proposal
it had previously rejected; in such circumstances, there
would be no point for the agents to engage in dialogue.
3 This last property is called dialectification in [15].
4 Expressing the rules of dialogue in terms of observable linguistic behaviour is called
externalization in [15].
12. System Simplicity: The locutions and rules of a dialectical
system should be as simple as possible, consistent with
the eleven criteria above. In particular, each locution should
serve a specific and stated function in the dialogue, and the
protocol rules should lead to efficient achievement of the dialogue
purposes.
13. Computational Simplicity: A dialectical system should be
designed to minimize any computational demands on its par-
ticipants, and on the system itself, consistent with the twelve
criteria above.
It is important to note two criteria we have not included here.
We have not specified that dialectical systems should be realistic
representations of some human dialogue, as we see no reason why
agent interactions should necessarily adopt human models of inter-
action. Indeed, dialectical systems may be applied to agent dialogues
which humans do not, or, even, could never undertake, such
as simultaneous negotiations over multiple products with hundreds
of participants. Secondly, we have not stated that the rules of a
dialectical system should require that the participants use particular
rules of inference (such as Modus Ponens), particular logics
or particular decision-making procedures, or that the participating
agents satisfy some criterion of rational behaviour, such as acting
to maximize expected utility. Insisting on such rules and criteria is
contrary to the notion of agent autonomy we assumed at the outset.
In addition to the thirteen principles listed above, there may be
further desiderata appropriate for specific types of dialogue. For in-
stance, for dialogues undertaken to negotiate a division of a scarce
resource, it may be considered desirable that outcomes are Pareto
optimal, i.e., that any other outcome leaves at least one participant
worse off [31]. Because we assume agents are free and willing
participants in a dialogue, acting under no duress, then any agreed
outcome to a negotiation dialogue will satisfy this particular cri-
terion, if certain of the above desiderata are met. We present the
result formally, so as to make clear the assumptions needed:
Proposition: Suppose two or more agents, each of which is purely
self-interested and without malice, engage freely and without duress
in a negotiation dialogue, i.e., a dialogue to agree a division of
some scarce resource. Suppose these agents use a dialogue protocol
which satisfies desiderata 2, 4, and 5, and that this dialogue
is conducted with neither time constraints nor processing-resource
constraints. Suppose further that their negotiation dialogue achieves
resolution, i.e., they agree on a division of the resource in question.
Then the outcome reached is Pareto Optimal.
Proof. Suppose that the outcome reached, which we denote by X ,
is not Pareto Optimal. Then there is another outcome, Y , which
leaves at least one agent, say agent a, better off, while all other
agents are no worse off. Then, it behooves agent a to suggest Y
rather than X for agreement by the participants, since agent a (like
all participants) is self-interested. If Y is suggested by agent a, the
other agents will at least be indifferent between Y and X , because
they are no worse off under Y and may be better off; so, being
without malice, the others should support proposal Y over X in
the dialogue. Now, the only reasons agent a would not suggest Y
would be because of: resource-constraints precluding the identification
of Y as a better outcome; time-constraints precluding the
making of the suggestion of Y in the dialogue; constraints imposed
on agent a by the protocol itself, e.g., rules precluding that particular
agent making suggestions; or social pressures exerted by other
agents on agent a which prevent the suggestion of Y being made.
Each of these reasons contradicts an assumption of the proposition,
and so X must be Pareto Optimal. 2
Of course, if the agents are not purely self-interested, or not free
of duress, or if they enter the discussion under constraints such as
resolution deadlines, then any agreement reached may not be Pareto
optimal. Since most agents in most negotiation dialogues will be
subject to resource- and time-constraints, Pareto optimality may
be seen as a (mostly) unachievable ideal for agent negotiation dia-
logues. An interesting question would be the extent to which any
given negotiation dialogue outcome approximates a Pareto optimal
outcome.
Finally, it is important to note that these desiderata, particularly
numbers 6 (Clarity of Argumentation Theory) and 11 (Enablement
of Self-Transformation), express a particular view of joint decision-making
by autonomous entities. Political theorists distinguish rational-choice
or marketplace models from deliberative democracy
models of social and public decision-making [6]. Rational-choice
models assume that each participant commences the decision-process
with his or her beliefs, utilities and preferences fully formed
and known (at least to him/herself); each participant then chooses
between (i.e., votes for or against) competing proposals on the basis
of his or her own beliefs and preferences. Such a model does
not allow for beliefs and preferences to be determined in the course
of the interaction, nor for participants to acquire a group view of
the issues involved in the decision, for instance, the wider social
consequences of individual actions [28]. In contrast, deliberative
democracy models of joint decision-making emphasize the manner
in which beliefs and preferences are formed or change through the
very process of interacting together, with participants undergoing
what has been called self-transformation [12, p. 184]. In a rational
decision-process, this transformation occurs by the sharing of
information, by challenging and defending assertions, by persua-
sion, and by joint consideration of the relevant issues - i.e., by
argument and debate. Because we assume that software agents are
autonomous, then such argument will be required to convince other
agents to adopt specific beliefs and to commit to specific intentions;
we believe, therefore, that a society of autonomous agents is best
viewed as a deliberative democracy, and not as simply a market-place

Comparison with Game Theoretic Models
At this point it is worth discussing the relationship of argumentation
protocols to work on game-theoretic approaches to negotiation, of
which perhaps the best known examples are [19, 29]. An example
of the kind of issue investigated in this work is how agents with
tasks to carry out in some environment can divide the tasks amongst
themselves to their mutual betterment - the task oriented domains
of [29, pp.29-52]. The key abstraction in this work is that the utility
of possible deals in the domain of negotiation (whatever agents are
negotiating over) can be assessed for any individual agent.
Perhaps the greatest attraction of game-theoretic approaches to
negotiation is that it is possible to prove many desirable features
of a given negotiation protocol. Examples of such properties include
[31, p.204]:
1. Maximising social welfare. Intuitively, a protocol maximises
social welfare if it ensures that any outcome maximises
the sum of the utilities of negotiation participants. If the utility
of an outcome for an agent was simply defined in terms
of the amount of money that agent received in the outcome,
then a protocol that maximised social welfare would maximise
the total amount of money "paid out."
2. Pareto efficiency. As discussed above.
3. Individual rationality. A protocol is said to be individually
rational if following the protocol - "playing by the rules"
- is in the best interests of negotiation participants. Individually
rational protocols are essential because, without them,
there is no incentive for agents to engage in negotiations.
4. Stability. A protocol is stable if it provides all agents with
an incentive to behave in a particular way. The best-known
kind of stability is Nash equilibrium.
5. Simplicity. A "simple" protocol is one that makes the appropriate
strategy for a negotiation participant "obvious". That
is, a protocol is simple if using it, a participant can easily
(tractably) determine the optimal strategy.
6. Distribution. A protocol should ideally be designed to ensure
that there is no "single point of failure" (such as a single
arbitrator), and so as to minimise communication between
agents.
It is worth comparing our desiderata with these. We do not explicitly
state maximising social welfare, as there is no general notion
of this in dialogue games. As we demonstrated above, our criteria
imply Pareto optimal outcomes. Individual rationality amount
to our criterion of individual purpose: an agent cannot be forced to
participate. We do not assume stability, but we do assume that there
in an incentive to resolve the dialogue, i.e., that it is in an agent's
interests to participate in the successful conclusion of the dialogue.
We explicily assume simplicity. Finally, we do not explicitly consider
distribution.
One of the best-known results in the area of game-theoretic negotiation
is Nash's axiomatic approach to bargaining, an attempt to
axiomatically define the properties that a "fair" outcome to negotiation
would satisfy - a desideratum, in effect, for negotiation [29,
pp.50-52]. The properties he identified were: (i) individual rationality
(a participant should not lose from negotiation); (ii) Pareto
invariance with respect to linear
utility transformations (e.g., if one agent counts utility in cents,
while the other counts it in dollars, it should make no difference
to the outcome); and (v) independence of irrelevant alternatives.
Nash proved that mechanisms that guarantee an outcome that maximises
the product of the utilities of participant agents satisfy these
criteria, and moreover, are the only mechanisms that satisfy these
criteria. We have begun working on the formalisation of dialogue
games [25, 26], with the goal of making these desiderata formal.
As a next step, it would be interesting to determine the extent to
which Nash's results transfer to our dialogue game framework.
3. DIALOGUE GAME PROTOCOLS
In this section, we examine three recent proposals for dialogue
game protocols against the desiderata presented above. The three
protocols, which are representative of the literature, have been selected
because they concern three different types of agent interac-
tions. However, not all elements of these protocols are fully speci-
fied, thus making it impossible to assess them against some of the
desiderata. In these cases, we write: Unable to assess.
3.1 A negotiation dialogue
We first consider a dialogue-game protocol for agent negotiation
dialogues proposed by Amgoud, Parsons and Maudet in [3], drawing
on the philosophical dialogue game DC of [23]. 5 This agent interaction
protocol comprises seven distinct locutions: assert, ques-
tion, challenge, request, promise, accept and refuse, and these can
5 This dialogue game was designed to enable persuasion dialogues which would preclude
circular reasoning.
be variously instantiated single propositions; arguments for
propositions (comprised of sets of propositions); or certain types of
implication. For example, the locution promise(p ) q) indicates a
promise by the speaker to provide resource q in return for resource
p. Arguments may be considered to be tentative proofs, i.e., logical
inferences from assumptions which may not all be confirmed.
The syntax for this protocol has only been provided for dialogues
between two participants, but could be readily extended to more
agents.
Following [14], when an agent asserts something (a proposition,
an argument, or an implication), this something is inserted into
a public commitment store accessible to both participants. Thus,
participants are able to share information. In [3], the protocol was
given an operational semantics in terms of a formal argumentation
system. In this semantics, an agent can only utter the locution as-
sert(p), for p a proposition, if that agent has an acceptable argument
for p in its own knowledge base, or in its knowledge base combined
with the public commitment stores. (Acceptable arguments
are those which survive attack from counter-arguments in a defined
manner.) The semantics provided, however, is not sufficient for
automated dialogues.
1. Stated Dialogue Purpose: The protocol is explicitly for negotiation
dialogues, but the syntax does not require the participants
to state the purpose(s) of the specific negotiation
dialogue undertaken.
2. Diversity of Individual Purposes: This is enabled.
3. Inclusiveness: There do not appear to be limitations on which
agents may participate.
4. Transparency: The protocol rules are transparent, and the
authors present the pre-conditions and post-conditions of each
locution.
5. Fairness: Locutions are only given for one participant (Pro-
ponent), with an implicit assumption that they are identical
for the other (Contestor).
6. Clarity of Argumentation Theory: The definitions of protocol
syntax and semantics assume an explicit theory of argumentation

7. Separation of Syntax and Semantics: The syntax is defined
in terms of the argumentation theory semantics, but could be
readily defined separately.
8. Rule-Consistency: The rules appear to be consistent.
9. Encouragement of Resolution: The protocol does not appear
to discourage resolution of the negotiation.
10. Discouragement of Disruption: Disruption is not discour-
aged, as there are no rules preventing or minimizing this be-
haviour. For instance, there are no rules precluding the repeated
utterance of the same locution by an agent, although
there is such a condition in the argumentation semantics given
for the dialogue protocol. 6
11. Enablement of Self-Transformation: Self-transformation
is not enabled. Agents may add to their knowledge base from
the commitment stores of other participants, but there appears
to be no mechanism for their knowledge base to change
or to diminish. Because transformation is not enabled, there
are no retraction locutions.
6 The dialogue game DC also lacks such a rule [23].
12. System Simplicity: There do not appear to be extraneous
locutions.
13. Computational Simplicity: Unable to assess. The computational
complexity of the semantic argumentation mechanism
may be high.
We believe the key weakness of this protocol is the absence of
self-transformation capability. The protocol also makes several implicit
assumptions, which may limit its applicability. Firstly, the
protocol assumes the interaction is between agents with fixed (al-
though possibly different) knowledge bases and possibly divergent
interests. Secondly, the absence of rules precluding disruptive behaviour
and rules for termination conditions, of an explicit statement
of objectives and of formal entry and exit locutions suggest
an implicit assumption that the participants are rational and share
some higher goals. Thirdly, although the semantic argumentation
framework allows agents to hold internally preferences regarding
arguments, the dialogue protocol does not allow for these to be expressed
in the dialogue; nor are degrees of belief or acceptability in
propositions and arguments expressible. Allowing such expression
should increase the likelihood of successful resolution of a negotiation
dialogue. For example, this protocol does not permit the
making of tentative suggestions - propositions uttered for which
the speaker does not yet have an argument.
3.2 A persuasion dialogue
We next consider the protocol proposed by Dignum, Dunin-Ke-p-
licz and Verbrugge [8] for the creation of collective intention by
a team of agents. The protocol assumes that a team has already
been formed, and that one agent, an initiator or proponent, seeks
to persuade others (opponents) in the team to adopt a group belief
or intention. For this dialogue, the authors adapt the rigorous persuasion
dialogue-game of [35], which is a formalization of a rigorous
persuasion dialogue in philosophy. Such dialogues involve
two parties, one seeking to prove a proposition, and one seeking to
disprove it. 7 The protocol presented by Dignum et al. includes
seven locutions: statement, question, challenge, challenge-with-
statement, question-with-statement and final remarks; these last in-
clude: "quit" and "won". The statements associated with challenges
and questions may be concessions made by the speaker.
1. Stated Dialogue Purpose: The protocol is explicitly for a
persuasion dialogue when an initiating agent seeks to "estab-
lish a collective intention within a group" [8, p. 313]. The
syntax requires the initiator to state explicitly the intention it
desires the group to adopt.
2. Diversity of Individual Purposes: The protocol assumes a
conflict of objectives by the participants, but not agreement.
3. Inclusiveness: There do not appear to be limitations on which
agents may participate.
4. Transparency: The protocol rules are transparent to the participating
agents. However, they are not yet fully specified,
since the authors do not articulate the pre-conditions and
post-conditions of each utterance, or all the rules governing
their use.
5. Fairness: Following [35], the protocol rules are asymmet-
rical: the initiator has different rights and obligations from
opponents. However, these differences are known to the participants

7 Note that the persuasion dialogues of [35] deal only with beliefs and not intentions.
6. Clarity of Argumentation Theory: The critical persuasion
dialogues for which the dialogue-game formalism [35] was
developed are idealizations of human dialogues, used by philosophers
to study fallacious reasoning. This underlying argumentation
theory is not stated explicitly in [8], nor is it
self-evidently appropriate for agent interactions.
7. Separation of Syntax and Semantics: The locutions and
syntax of the dialogue are not fully articulated. A partial
operational semantics is provided in terms of the beliefs and
intentions of the participating agents. To the extent that the
syntax and semantics are specified, they appear to be defined
separately.
8. Rule-Consistency: Unable to assess this, as the rules are not
fully articulated.
9. Encouragement of Resolution: The argumentation theory
underlying the protocol assumes the participants have contrary
objectives, which is not necessarily the case. By assuming
antagonism where this is none, the protocol may discourage
resolution.
10. Discouragement of Disruption: Disruption is not discour-
aged, as there are no rules preventing or minimizing this be-
haviour. For instance, there are no rules precluding the repeated
utterance of the same locution by an agent.
11. Enablement of Self-Transformation: Self-transformation
is enabled. However, because the syntax and semantics are
not fully articulated, it is not clear how this is achieved. 8 In
addition, this protocol does not permit degrees of belief or
acceptability to be expressed, nor does it permit retractions
of prior statements.
12. System Simplicity: There do not appear to be extraneous
locutions. However, following [35], participants may only
speak in alternating sequence, and the rules of the dialogue
are quite strict.
13. Computational Simplicity: Unable to assess. The computational
complexity of the semantic mechanism may be high,
as the authors concede.
This protocol is difficult to assess against the desiderata because
the locutions, rules of syntax and the semantics are not fully articu-
lated. In addition, the authors present no case for using the rigorous
persuasion dialogue game adapted from [35] in the agent domain.
This game embodies an explicit theory of argumentation which is
not necessarily appropriate for agent dialogues. In particular, the
theory assumes participants are engaged in a critical persuasion,
and thus have conflicting objectives (namely to prove or disprove a
proposition); consequently the rules and locutions are stricter than
most people would consider appropriate for an ordinary (human
or agent) persuasion dialogue. Participants must speak in alternating
sequence, for example. Moreover, the rigorous persuasion dialogue
is based on the dialogue games of [22], originally designed
as a constructive proof-theory for logical propositions. While such
games can be used to construct, step-by-step, an argument for a
proposed group intention, this would seem a singularly inefficient
means of persuasion. Allowing agents to express a complete argument
for a proposal in one utterance, as Amgoud et al. permit
8 An agent asked by an initiator to adopt an intention which conflicts with an existing
intention may challenge the initiator to provide a proof for the proposed intention, but
the authors do not indicate when and how that proof leads to a revision of the existing
intention [8, Section 4.2.4].
in the negotiation protocol assessed above, would seem far more
efficient.
3.3 An inquiry dialogue
We now consider a dialogue game protocol proposed by McBurney
and Parsons for inquiry dialogues in scientific domains [24].
This presents 30 locutions, which enable participants to propose,
assert, question, accept, contest, retract, and refine claims, the arguments
for them, the assumptions underlying and the rules of inference
used to derive these arguments, and the consequences of
claims. The protocol is based on a specific philosophy of science,
due to Feyerabend and Pera, which stresses the dialogical nature of
scientific knowledge-development. In addition to the protocol syn-
tax, a game-theoretic semantics is presented, linking arguments in
the dialogue after finite times with the long-run (infinite) position
of the dialogue. 9
1. Stated Dialogue Purpose: There is no stated dialogue purpose

2. Diversity of Individual Purposes: This is enabled.
3. Inclusiveness: There do not appear to be limitations on which
agents may participate.
4. Transparency: The protocol rules are transparent, and the
authors present the pre-conditions and post-conditions of each
locution, along with rules governing the combination of locutions

5. Fairness: The rules treat all participants equally. Utterance
of specific statements may incur obligations on the agent
concerned.
6. Clarity of Argumentation Theory: The protocol conforms
explicitly to a specified philosophy of scientific discourse,
uses Toulmin's well-known model of an individual argument
[34], and adheres to most of the principles of rational human
discourse proposed by Alexy and Hitchcock [1, 15]. The
conformance of the protocol is demonstrated formally.
7. Separation of Syntax and Semantics: These are defined
separately.
8. Rule-Consistency: The rules appear to be consistent.
9. Encouragement of Resolution: The protocol does not appear
to discourage resolution, but no rules for termination are
provided. Because this is a model for scientific dialogues, it
is assumed to be of possibly-infinite duration.
10. Discouragement of Disruption: The rules prohibit multiple
utterances of the same locution, but not other forms of
disruption.
11. Enablement of Self-Transformation: Self-transformation
is enabled. Assertions may be retracted and qualified. In
addition, degrees of belief in propositions and rules of inference
may be expressed. However, the mechanisms of self-
transformation internal to an agent are not presented.
12. System Simplicity: With locutions, the protocol is not
simple.
9 The purpose of this semantics is to assess to what extent a finite shapshot of a debate
is representative of the long-run counterpart, in order to assess the likelihood of
dialogues conducted under the protocol finding the answer to the question at issue.
13. Computational Simplicity: Unable to assess.
The semantics provided for this protocol is not an operational
semantics, and no assumptions are made concerning the internal
architectures of the participating agents. Deciding what locutions
to utter in a dialogue under this protocol may be computationally
difficult for a participating agent, particularly if the number of participants
is large and the topics discussed diverse. The absence of
a stated dialogue purpose means that any topic may be discussed at
any time. This is a significant weakness of the protocol.
4. THE FIPA ACL
Finally, by way of comparison, we consider the Agent Communications
Language of FIPA, the Foundation for Intelligent Physical
Agents [11]. The FIPA ACL standard essentially defines a standard
format for labelled messages that agents may use to communicate
with one-another. The standard defines 22 distinct locutions,
and these have been provided with an operational semantics using
speech act theory [20]. The semantics of the language is defined using
pre- and post-condition rules, where these conditions define the
mental state of participants of communication - their beliefs and
intentions. This semantics links utterances in the dialogue to the
mental states of the participants, both preceding utterance of each
locution, and subsequent to it. All the locutions in the FIPA ACL
are ultimately defined in terms of inform and request primitives.
The FIPA ACL standard is a generic agent communication pro-
tocol, and is not based on a dialogue game. However, the various
dialogue-game protocols have all been proposed for agent interactions
negotiation, persuasion, etc - for which the FIPA ACL
could potentially also be used. It is therefore of interest to see how
the FIPA ACL compares to these protocols when assessed against
the desiderata above.
1. Stated Dialogue Purpose: The ACL is intended primarily
for purchase negotiations, and use of the locution cfp -
standing for Call For Proposal - can initiate a negotiation
dialogue with a stated purpose. There does not appear to be
means to state the purpose of other types of dialogue, e.g.,
information-seeking or persuasion dialogues.
2. Diversity of Individual Purposes: This is enabled.
3. Inclusiveness: There do not appear to be limitations on which
agents may participate.
4. Transparency: The ACL rules are transparent, and the definitions
present the pre-conditions and post-conditions of each
locution.
5. Fairness: The rules treat all participants equally.
6. Clarity of Argumentation Theory: There is no explicit underlying
argumentation theory for the FIPA ACL. Implicitly,
the argumentation model is an impoverished one. Participating
agents, for example, have only limited means to question
or contest information given to them by others, i.e., via the
not-understood locution. Moreover, the rules provide speakers
uttering such challenges with no rights to expect a defence
of prior assertions by those who uttered them.
7. Separation of Syntax and Semantics: The syntax is defined
in terms of the semantics, so the two are not separated. For
example, the pre-conditions of the inform locution include a
sincerity condition: the speaker must believe the argument of
this locution to be true before uttering the locution.
8. Rule-Consistency: The rules appear to be consistent.
9. Encouragement of Resolution: The FIPA ACL does not appear
to discourage resolution, but no rules for dialogue termination
are provided.
10. Discouragement of Disruption: There are no rules which
explicitly preclude disruptive behaviour, although the rationality
conditions incorporated in the semantics may limit such
behaviour.
11. Enablement of Self-Transformation: Self-transformation
is limited. The semantics imposes sincerity conditions on ut-
terances, and so agents may only assert what they sincerely
believe to be true. However, there are no locutions for retraction
of prior assertions, and no means to express degrees of
belief or to qualify assertions.
12. System Simplicity: The locutions include both substantive
locutions, e.g., accept-proposal, and procedural locutions,
e.g., propagate, which asks the recipient to forward the message
contents to others. The language would be simpler if
these were treated as different classes of locution.
13. Computational Simplicity: The FIPA ACL essentially just
defines a standard format for messages, and so it is hard to
assess in general the complexity of its use. However, because
agents must check the sincerity condition of inform locutions
before uttering these, they require an internal proof mecha-
nism; this will be in the first-order modal logic of the FIPA
ACL semantics [20], which is at best semi-decidable.
The implicit model of joint decision-making underlying the FIPA
ACL is a rational choice one. As explained in Section 2, this model
has no place for self-transformation in the course of the interac-
tion, and hence gives no value to argumentation activities such as
information-seeking, persuasion, inquiry or joint deliberation. The
rational-choice model may be appropriate for agent purchase negotiation
dialogues, although marketing theorists would argue differ-
ently, since their models of consumer behaviour typically assume
that consumer preferences may only be finalized during the purchase
decision process and not before, e.g., [21]. However, the
rational-choice model, as we argued earlier, is not at all appropriate
for other forms of agent dialogue, such as joint determination of
plans of action or joint inquiries after truth.
5. DISCUSSION
Designer's Checklist
The experience of developing the list of desiderata presented above
and applying them to the three dialogue game protocols have led us
to formulate a list of guidelines for designers of such protocols, as
follows:
G1 The protocol should embody a formal and explicit theory of
argument.
G2 The rules for the protocol should ensure that the reason(s) for
conducting the dialogue are stated within the dialogue at its
commencement.
G3 The protocol should include locutions which enable participants
to:
G3.1 formally enter a dialogue
G3.2 request information
G3.3 provide information
G3.4 request arguments and reasons for assertions
G3.5 provide arguments and reasons for assertions
G3.6 challenge statement and arguments
G3.7 defend statement and arguments
retract previous assertions
G3.9 make tentative proposals
express degrees of belief in statements
G3.11 express degrees of acceptability or preferences regarding
proposals
G3.12 formally withdraw from a dialogue.
G4 The protocol syntax should be defined in observable terms, so
this its conformance can be verified without reference to internal
states or mechanisms of the participants.
G5 The rules of the protocol should seek to preclude disruptive
behavior.
G6 The rules of the protocol should indicate circumstances under
which a dialogue terminates.
G7 The rules of the protocol should identify any difference in formal
roles and the rights and duties pertaining to these.
These guidelines may be viewed as a checklist for designers and
users of agent interaction protocols involving argumentation. To
our knowledge they are the first such design guidelines proposed
for agent dialogue game protocols.
Conclusions
In this paper we have presented the first list of criteria by which to
assess a dialogue-game protocol for agent interactions. These thirteen
desiderata were developed after consideration of: economic
and computational criteria recently proposed for the assessment of
automated auction and negotiation mechanisms; theories of social
and public decision-making in argumentation theory and political
philosophy; and recent research in designing and studying agent
communications languages and interaction protocols.
We have applied these thirteen desiderata to three dialogue game
protocols from the literature, for agent dialogues involving nego-
tiation, persuasion and mutual inquiry, respectively. Interestingly,
each protocol was found to be weak in at least one important as-
pect. The negotiation protocol did not permit agents to express
changes in their beliefs or preferences in the course of the dialogue
(what political theorists call self-transformation), while the persuasion
dialogue implicitly drew on a theory of argumentation we believe
to be inappropriate for the agent domain. The inquiry dialogue
was the only one to make explicit the argumentation theories upon
which it is based, but it permitted discussion to range over any and
all topics simultaneously, thereby limiting its practical usefulness.
We also considered the Agent Communications Language (ACL)
of FIPA, for comparison purposes. The key weaknesses of FIPA
ACL, relative to these desiderata, were found to be its limited support
for formal argumentation and for self-transformation by par-
ticipants. These findings were not surprising, since its designers
did not seek to embody a theory of argumentation, and because it
appears to express a rational-choice (or marketplace) view of agent
society, rather than a deliberative democracy view. In our opin-
ion, these weaknesses preclude the use of FIPA ACL beyond the
purchase negotiation dialogues it was designed for.
In future work, we aim to formalize these desiderata and thus
be in a position to prove formally the properties of dialogue game
protocols, such as those assessed in Section 3.
6.



--R

A theory of practical discourse.
Modelling dialogues using argumentation.

A method for the computational modelling of dialectical argument with dialogue games.
The limits of the dialogue model of argument.
Deliberative Democracy: Essays on Reason and Politics.


Communication and Fallacies: A Pragma-Dialectical Perspective
Environmental risk and democratic process: a critical review.

The Deliberative Practitioner: Encouraging Participatory Planning Processes.
Denotational semantics for agent communication languages.

Some principles of rational mutual inquiry.
A framework for deliberation dialogues.
Dialogue Models for Inquiry and Transaction.
The problem of retraction in critical discussion.
Strategic Negotiation in Multiagent Environments.
Agent communication languages: The current landscape.
Marketing Models.
Dialogische Logik.

Representing epistemic uncertainty by means of dialectical argumentation.
Games that agents play: A formal framework for dialogues between autonomous agents.
A geometric semantics for dialogue-game protocols for autonomous agent interactions
A dialogue-game protocol for agent purchase negotiations
The argumentation theorist in deliberative democracy.
Rules of Encounter: Designing Conventions for Automated Negotiation among Computers.
Logic agents
Distributed rational decision making.
A framework for argumentation-based negotiation
A social semantics for agent communications languages.
The Uses of Argument.
Commitment in Dialogue: Basic Concepts of Interpersonal Reasoning.
Semantic issues in the verification of agent communication languages.
--TR
Rules of encounter
Distributed rational decision making
Denotational semantics for agent communication language
Representing Epistemic Uncertainty by Means of Dialectical Argumentation
Games That Agents Play
Semantic Issues in the Verification of Agent Communication Languages
Agent Communication Languages
A Social Semantics for Agent Communication Languages
A Framework for Argumentation-Based Negotiation
Agent Theory for Team Formation by Dialogue
A Dialogue Game Protocol for Agent Purchase Negotiations
Modeling Dialogues Using Argumentation

--CTR
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interaction protocols;FIPA;argumentation;dialogue games;agent communication languages