Scaffolding and the Insufficiency of the Intentional Stance as a
Conceptual Underpinning for Multiagent Systems
Albert Esterline
Dept. of Computer Science, North Carolina A&T State University
1601 East Market Street
Greensboro, NC 27411
esterlin@ncat.edu
Space restrictions confine discussion to representative
positions. We consider the criticisms of ethnomethodology
since they present a clear attack on the foundations of
multiagent research by questioning the role of plans, taken
in a very broad sense, in coordination. And we consider
Bratman’s analysis of plans since it is in clear contrast with
that of ethnomethodology, exposes the conceptual relations
among the notion of plans and other basic notions, and has
been endorsed by much of the multiagent community.
Abstract
The intentional stance, which has been a conceptual
underpinning of much work on multiagent systems, cannot
provide an account of coordinated behavior in terms that
can apply equally to humans and to agents. We suggest that
one must also include social scaffolding in the sense in
which “scaffolding” is used by Andy Clark. Critical aspects
of the relevant social scaffolding can be formulated in terms
of notions familiar from formalisms that have been applied
to software systems and to humans alike.
The Intentional Stance
Introduction *
The intentional stance finds its classical exposition in the
work of the philosopher Dennett (Dennett 1987). The
intentional strategy (adopting the intentional stance),
according to Dennett, is one of several strategies—such as
the physical strategy and the design strategy—we use to
predict the behavior of systems. When even the design
stance is impractical, there is still the intentional stance:
The key feature unifying agent research is viewing
computational entities as human-like. The theoretical step
justifying this is known as adopting the intentional stance.
This paper suggests, however, that the intentional stance
cannot provide an account of coordinated behavior in terms
that can apply equally to humans and to agents. What
further is required is what Andy Clark has called
scaffolding (Clark 1997): a complex world of physical and
social structures on which the coherence and analytic
power of human activity depends. Of interest here is social
scaffolding, critical aspects of which can be formulated in
terms of notions familiar from formalisms that have been
applied to software systems and to humans alike.
The next section reviews Dennett’s formulation of the
intentional stance, and the following section present’s
Bratman’s position on intentions and plans and considers
the contrasting findings of ethnomethodoloy.
For
scaffolding, we first consider obligations, suggesting that
speech acts, the normal way of establishing directed
obligations, be seen as joint actions modeled as handshakes
(in the process-algebraic sense). The consequences for a
theory of multiagent systems of the fact that specifications
establish obligations are investigated in the following
section, then common “knowledge” is identified as another
aspect of social scaffolding. After a brief caveat on using
modal logics in these contexts comes the conclusion.
… first you decide to treat the object … as a rational
agent; then you figure out what beliefs that agent
ought to have, given its place in the world and its
purpose. Then you figure out what desires it ought to
have, on the same considerations, and finally you
predict that this rational agent will act to further its
goals in the light of its beliefs. (p. 17)
Practical reasoning then lets you decide what the agent out
to do, which is what you predict. The intentional stance
gives shallow accounts of simple artifacts but, for complex,
versatile systems (e.g., chess-playing programs and,
paradigmatically, humans), it significantly constrains the
internal constitution attributed to the intentional system.
Intentions, Plans, and Criticisms of the AI
Planning Model
Critical components of this internal constitution for both
humans and agents are plans. Bratman (Bratman 1990),
whose work inspired BDI agent architectures, distinguishes
between a plan as a sort of recipe (as in traditional AI) and
*
Copyright © 2007, Association for the Advancement of Artificial
Intelligence (www.aaai.org). All rights reserved.
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a plan in the sense of having a plan as a mental state,
essentially an intention. Bratman distinguishes between
two kinds of pro-attitudes. Pro-attitudes that are merely
potential influencers of conduct include such things as
desires, but intentions (plans) are conduct controlling,
providing consistency constraints as they must be both
internally consistent and consistent with the agent's beliefs.
They should also have means-end coherence, being
elaborated enough for me to do what I now plan.
Intentions thus pose problems for deliberation hence
determine the relevance of options. They are thus stable
yet (for rationality) not irrevocable. So future-directed
intentions allow deliberation in advance of conduct and
support coordination by supporting expectations of their
successful execution.
A feature of plans/intentions
characteristic of limited agents is that they are partial:
details are left for later deliberation.
Suchman (Suchman 1987) criticized the notion of plans
as formal structures generating actions that predominates
in classical AI. Face-to-face human conversation is taken
as the baseline for assessing the state of human-computer
interaction; such conversation is found to be at odds with
the traditional planning model since it is not so much an
alternating series of actions as a joint activity continuously
engaging the participants. Suchman maintains that a plan
is not a generative mechanism but an artifact of reasoning
about actions, an abstraction over actions; a plan is a
resource for action, used retrospectively to justify a course
of action and before the fact to orient us. Suchman's work
heralded a body of naturalistic research by social scientists
into interaction in technology-supported activities.
Ethnomethodology and conversation analysis, the main
orientations here, “examine the ways in which participants
reflexively, and ongoingly, constitute the sense of
intelligibility of the ‘scene’ from within the activities in
which they are engaged” (Heath and Luff 2001, p. 19). It
is generally found that “the accomplishment of complex
tasks … is ‘ongoingly’ co-ordinated with the actions of
others” (Heath and Luff 2001, p. 20).
Bratman’s position, in contrast, is that a plan as a
resource is a recipe, which, if embraced by intention,
becomes a plan effective of action. In fact, the notion of
expectations based on intentions comes closer to the core
of multiagent systems and group activity than does that of
an inert recipe. An intention, however, is an attitude,
hence private; for multiagent systems, we consider
communication acts, which are public. So we shift from
intentions to obligations. “Obligation” and ”commitment”
are related, but the latter can also describe an attribute of a
person (a resolve), so we prefer “obligation.”
might be obligated to teach at 10:00 AM and also be
obligated to appear in court at 10:00 AM. One of the
prima facie obligations defeats the others, becoming the
actual obligation. And what was once an actual obligation
may be defeated by a new obligation. Whereas having an
intention/plan and its reconsideration are up to the agent,
being under an obligation and which prima facie
obligations defeat others are objective. Thus, one can have
an obligation to A yet not intend to A; conversely, one can
have an intention without the corresponding obligation.
Yet there is a close relation between intentions and
obligations since normally one intends to discharge one’s
obligations—otherwise, obligations would be pointless.
Thus, obligations serve much the same functions
Bratman identifies for intentions.
They support
coordination by supporting expectations of their successful
discharge because they are normally stable, and they drive
means-end reasoning. Imposing an obligation on oneself
(e.g., promising) or another (e.g., commanding) allows
deliberation in advance of conduct. But obligations have a
more meager requirement of means-end coherence, being
more abstract and one more step away from conduct. They
are also conduct controllers, again one more step away
from conduct, although they are not attitudes. Focusing on
obligations gives an angle on the intentional stance that
emphasizes ideal behavior.
Speech Acts, Handshakes, and Processalgebraic Plans
Focusing on obligations emphasizes speech acts, the
normal way to establish directed obligations, where there is
an obligor (subject to the obligation) and an obligee (to
whom the obligation is owed). For example, a promise
establishes the speaker as the obligor and the addressee as
the obligee, while a command reverses these roles.
As contemporary analysis of face-to-face conversation
emphasizes the active role of addressees (e.g., nods), we
view speech acts as joint actions. Since the agents
involved in a joint action must time their contributions so
that each contributes only when all are prepared, a good
formal model for a speech act is a handshake in a process
algebra (e.g., the S-calculus (Milner 1999), a joint
communication action that happens only when both parties
are prepared. In a process algebra, terms denote processes,
and combinators apply to processes to form more complex
processes. Combinators typically include alternative and
parallel composition as well as a prefix combinator that
forms a process from a given process and a name. A
handshake results in an action identified by the prefix of
the selected alternative, and the resulting process consists
of only the selected alternative with its prefix removed.
Names come in complementary pairs, and parallel
processes may handshake only if they have alternatives
with complementary prefixes, that is, only when both are
prepared. A process can evolve only by handshaking,
which synchronizes components’ behaviors.
Obligations
Obligations, like intentions, are normally stable, and they
are defeasible (overridable). There is interplay between the
consistency constraint on obligations and their
defeasibility. We may have several prima facie obligations
that clash in that not all can be discharged. For example, I
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read “agent i knows that M”. EGM, read as “everyone (in G)
knows that M,” is defined as K1M š K2M š … š KnM. Let
EGk be the EG operator iterated k times. Then “it is
common knowledge in group G that M,” in symbols, CG M,
is defined with the infinite conjunction EG1 M š EG2 M š …
š EGi M š …, that is, everyone knows that M, everyone
knows that everyone knows that M, and so on, for
arbitrarily deep nestings of “everyone knows that.”
Common knowledge is a prerequisite for coordinated
action. For example, traffic lights would not work unless it
were common knowledge that green means go, red means
stop, and that lights for opposite directions have different
colors. If this were not so, we would not confidently drive
through a green light. In a standard epistemic logic (such
as S5) augmented with the operators defined above, it is
easy to show that, if everyone in G agrees that \, then the
agreement is common knowledge (Fagan et al. 2003). It
can also be shown formally that coordination implies
common knowledge.
Characterizing common knowledge in terms an infinite
conjunction is labeled the iterate approach by Barwise
(Barwise 1989), who identifies two other approaches. In
the fixed-point approach (which eliminates the infinite
conjunction), we view CG M as a fixed-point of the function
(Fagan et al. 2003) f(x) = EG(M š x). Specifically, in
augmented S5, we can derive CG M œ EG (M š CG M). The
third approach (which we formulate following (Clark and
Carlson 1982)) is the shared situation approach. For this,
where A and B are rational, we may infer common
knowledge among A and B that M if
1. A and B know that some situation V holds.
2. V indicates to both A and B that both A and B know
that V holds.
3. V indicates to both A and B that M.
Barwise concludes that the fixed-point approach
(essentially implied by the shard situation approach) is the
correct analysis of common knowledge, and that common
knowledge generally arises via shared situations. It is
tempting to view the iterate approach as characterizing
how common knowledge is used, but progress through the
ever deeper nestings of “everyone knows that” is blocked
by doubt at some level.
Barwise points out that common knowledge is not
properly knowledge. Knowing that M is stronger than
carrying the information that M since it relates to the ability
to act. Also (Fagan et al. 2003), if we considers common
knowledge as a disposition of individuals, we face the
paradox of logical omniscience since the standard possibleworlds semantics of epistemic logic requires that we know
all logical consequences of what we know. And a thesis
of standard epistemic logic states that information becomes
shared in the required sense at the same time for all agents
sharing it, no surprise since all the agents are involved in
the circularity.
Barwise concludes that common
knowledge (as per fixed the-point approach) is a necessary
but not sufficient condition for action and is useful only
when arising in a shared situation that, if maintained,
“provides a stage for maintaining common knowledge.”
Plans as process-algebraic terms, then, provide an
analysis in line with conversation and are appropriate for
multiagent systems. Such terms can represent the structure
of a joint activity. What is effective of the activity,
however, are the intentions of the agents to participate and,
at a more abstract level, the obligations that spawn these
intentions and that arise as a result of communication
actions within the activity itself.
Specifications and Obligations
Khosla and Maibaum (Khosla and Maibaum 1987) point
out that a specification of a software system establishes
obligations on the behavior of the specified system. When
the system is an agent, aspects of the specification that
relate to its behavior within a multiagent system are the
constraints on the sequences of communication acts it may
perform. These can be seen as sequences of speech acts
establishing and discharging obligations. Violation
warrants a sanction, such as taking a compensating action.
The expectations agents have of the behavior of other
agents derives from the others’ specifications. So we view
the general notion of an agent at the specification level.
That this is the appropriate level is also suggested by the
fact that there are many ways to realize a handshake, and
picking out the handshakes is part of characterizing the
situation. (For this point regarding adjacency pairs in
conversation analysis, see (Heritage 1984, p. 302).) The
important thing for us is that we can specify that certain
handshakes are required and obligate other handshakes no
matter how these handshakes are realized.
AS a specification can be implemented any number of
times, the agent abstraction is for a role. A role can be
seen as a resource since behavior is stated declaratively,
but once the role is assumed, obligations are assumed, and
it becomes effective of action. A role can introduce
special obligations, with an obligor but no oblige, which
tend to be more abstract than directed obligations. The
protocol specified by the environment introduces general
obligations, with neither an obligor nor an oblige, which
are conditions for any agent to be in the multiagent system.
So obligations, being public, are part of the social
scaffolding needed for cooperation. Obligations persist,
some (like many directed obligations) until they are
discharged, while others (special obligations) attach to a
role or generally hold of anyone or anything participating
in the given society, but almost all can be defeated in some
circumstances by other obligations.
Common Knowledge
While obligations are effective of coordinated activity,
common knowledge is a necessary condition for it. To
explain this concept (Fagan et al. 2003), let G be a group of
n agents, denoted by ordinals, G = {1, 2, …, n}. We
introduce n modal operators Ki, 1 d i d n, where Ki M is
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Common knowledge, rather than being a disposition
mysteriously shared by a group, is a feature of the social
scaffolding. If we analyze situations rigorously, then
fixed-points seem appropriate, and logical omniscience and
simultaneity requirements regarding the scaffolding are
acceptable.
Barwise addresses situations, but the
scaffolding and the common “knowledge” it supports are
much more embracing. H. H. Clark and Carlson (Clark
and Carlson 1982) identified three “co-presence heuristics”
giving rise to different kinds of shared “situations”. Two,
physical co-presence and linguistic co-presence, properly
relate to situations, but the third, community membership,
is not temporally or spatially restricted, and they suggest
that the other two heuristics presuppose it. The scaffolding
in fact allows us to escape spatial and temporal bounds and
to overcome simultaneity requirements, as the use of
writing notably attests.
action; rather than something dispositional, it is seen as a
feature of the social scaffolding, a view that mollifies
several paradoxes.
For multiagent systems, much of the scaffolding consists
of protocols that are common knowledge among
participants and guarantee establishment of the required
common knowledge. This scaffolding is so transparent
that it can be mistaken for the activities it supports. For
example, a syntactically correct message with a
performative is a common characterization of speech act
but, in fact, is not an act of any kind. In fact, synchronous
communication (akin to human conversation) is more
primitive conceptually than asynchronous communication,
although the latter is more tractable given the physical
scaffolding supplied by the computational infrastructure. A
penetrating analysis of multiagent systems can be achieved
with a proper view of the relation between the scaffolding
and the coordinated activity it supports.
A Note on Modal Logics
References
Although the aspects of the social scaffolding we address
relate to concepts in some sense captured by modal logics,
these logics do not tell us how the concepts apply to
reality. We have suggested that epistemic logic, at least
regarding common “knowledge”, is really concerned with
the scaffolding. Deontic logics purportedly capture the
notions of obligation, permission, and prohibition.
Standard deontic logics relate to “ought to be” while
“ought to do” is perhaps more natural, and deontic
modalities relating to agency, as in “ought to bring-itabout-that,” are perhaps more revealing. Yet, while there
is no one correct way to analyze deontic notions, the
alternatives and the attempts to dispel the paradoxes
plaguing deontic logics sharpen our analytic tools.
Barwise, Jon. 1989. Chapter 9, On the Model Theory of
Common Knowledge, in The Situation in Logic. Stanford,
CA: CSLI.
Bratman, M. What is Intention? In Cohen, P. R., Morgan,
J., and Pollack, M.E. eds. 1990. Intentions in
Communication, Cambridge, MA.: The MIT Press, 15-31.
Clark, Andy. 1997. Being There: Putting Brain, Body, and
World Together Again. Cambridge, MA: The MIT Press.
Clark, H.H., and Carlson T.B. Speech Acts and Hearers’
Beliefs. In Smith, N. V. ed. 1982. Mutual Knowledge. New
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Dennett, D. C. True Believers: The Intentional Strategy
and Why It Works. In Dennett, D. C. ed. 1987. The
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Fagin, R., Halpern, J. Y., Moses, Y. and Vardi, M. Y.
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Heritage, J. 1984. Garfinkel and Ethnomethodology.
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Conclusion
To provide an account of coordinated behavior, one must
augment the intentional stance with social scaffolding,
critical aspects of which can be formulated in terms of
notions familiar from formalisms that have been applied to
software systems and to humans alike. We have reviewed
Bratman’s position on intentions and plans and considered
the contrasting findings of ethnomethodoloy, that complex
tasks are accomplished by ongoingly coordinating with the
actions of others. For scaffolding, we first considered
obligations, which provide most of the support for activity
attributed to intentions, but, being part of the scaffolding,
are effective of coordinated activity. We suggested that
speech acts, the normal way of establishing directed
obligations, be seen as joint actions modeled as
handshakes; then plans as process-algebraic terms provide
an analysis in line with conversation that is appropriate for
multiagent systems. We suggested that the general notion
of an agent is at the specification level and relates to roles,
which come with obligations. Finally, we addressed
common knowledge as a prerequisite for coordinated
Khosla, S. and Maibaum, T. S. E. The Prescription and
Description of State Based Systems. In Banieqbal, B.,
Barringer, H., and Pnueli, A. eds. 1987. Temporal Logic in
Specification. Berlin: Springer-Verlag, 243-294.
Milner, R. 1999. Communicating and Mobile Systems: The
S-calculus, Cambridge, UK: Cambridge University Press.
Suchman, L. A. 1987. Plans and Situated Actions: The
Problem of Human-Machine Communication. New York.
Cambridge University Press.
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