Published in Jennings, N. and O'Hare, G. (eds.), Foundations of Distributed Artificial Intelligence , Wiley, New York (1996), pp. 487-503
(Note: This is a manuscript version. Please quote only the published version.)
PHILOSOPHY AND DISTRIBUTED ARTIFICIAL INTELLIGENCE
The Case of Joint Intention
Raimo Tuomela
University of Helsinki
I PHILOSOPHY AND DAI
In current philosophical research the term 'philosophy of social
action' can be used - and has been used - in a broad sense to
encompass the following central research topics: 1) action occurring
in a social context; this includes multi-agent action; 2) joint
attitudes (or "we-attitudes" such as joint intention, mutual belief)
and other social attitudes needed for the explication and explanation
of social action; 3) social macro-notions, such as actions performed
by social groups and properties of social groups such as their goals
and beliefs; 4) social norms and social institutions (see Tuomela,
1984, 1995). The theory of social action understood analogously in a
broad sense would then involve not only philosophical but all other
relevant theorizing about social action. Thus, in this sense, such
fields of Artificial Intelligence (AI) as Distributed AI (DAI) and
the theory of Multi-Agent Systems (MAS) fall within the scope of the
theory of social action. DAI studies the social side of computer
systems and includes various well-known areas ranging from Human
Computer Interaction, Computer-Supported Cooperative Work,
Organizational Processing, Distributed Problem Solving to Simulation
of Social Systems and Organizations. Even if I am a philosopher with
low artificial intelligence I will below try to say something about
what the scope of DAI should be taken to be on conceptual and
philosophical grounds. (In the later sections of the paper the
central notion of joint intention will be the main topic - in order
to illustrate how philosophers and DAI-researchers approach this
issue.)
Let us now consider the relationship between philosophy - especially
philosophy of social action - and DAI. Both are concerned with social
matters and in this sense seem to have a connection to social science
proper. What kinds of questions should these areas of study be
concerned with? In principle, ordinary social science should study
all aspects of social life (in various societies and cultures), try
to describe it and create general theories to explain it.
Philosophers do not create specific social theories but their task in
this context is to create conceptual frameworks for the study of
social life, to investigate central social concepts, and to discuss
and critically evaluate different fundamental ideas and philosophical
views about social life and social systems. What is the task of DAI,
then? Broadly put, DAI should basically attempt to study - and to
create "design specifications" for - a social world of intelligent
robots; it should also be concerned with the actual technical
implementation of those specifications. As DAI is basically concerned
with intelligent agents it typically adopts rationality assumptions
which are stronger than those social scientists can plausibly make
(social scientists should also be interested in stupid, clumsy and
mistake-making agents), but because of the end of actually building
robots it will have to take into account the resource-boundedness of
intelligent agents. This concern with bounded resources may also
contrast with how philosophers - those prime idealizers - approach
these matters. My most general thesis in this paper is this:
Philosophy is in many ways highly relevant and important for DAI, and
- to some extent - vice versa . A sharp borderline
between these fields hardly exists, even if philosophers of social
action, qua philosophers, are interested in somewhat
different aspects of social action (and related matters). I argue
that, in principle, one of the tasks of philosophy is to specify the
research areas and problems of DAI on a general level (but only on a
general level). DAI need not be interested in simulating all the
inner mechanisms and processes that lead agents to do what they do in
a social setting, but it has to be concerned with what people achieve
by their actions - with "social outputs" - and various related public
matters including social reasons for action. This public output
aspect is something that philosophers should specify on a general
level: Philosophers ought to say what the output side of social life
- and the life of the community of intelligent robots - basically is,
or at least they ought to be able to sketch plausible scenarios about
it. Here we arrive at fundamental questions such as what general
reasons there are for people to form communities and societies at all
and what kind of conceptual problems are involved in answers to these
questions. This is a broad topic which I can only touch briefly
here.
Basically, agents have not only personal needs and wants but also
collective ones. Thus we may have as our collective goal to keep our
environment clean; no single agent alone can achieve that collective
goal. Or, to mention a small-scale collective or joint goal, we may
want to get deer meat, believing that cooperative joint hunting is
desirable or even necessary as compared with hunting separately or
alone. In this kind of context - especially when a collective goal or
"good" is to be achieved - collective action problems
are central. Such problems involve a conflict between individual and
collective interests, because there is an inbuilt incentive to
free-ride (e.g. in our example to litter or not to care). Collective
action problems typically have the structure of a Prisoner's Dilemma
game or a Chicken game.
As indicated, in social life there are central tasks that can be
solved only collectively: a) tasks requiring collective goals (e.g.
the provision of collective goods in the economist's sense); often
but not always there is a collective action problem here - conflict
between collective and individual rationality; b) individual goals
(tasks) which, however, require collective cooperation concerning the
means (personal goals thus can be satisfied only through collective
or joint action). Collective needs and collective goals over and
above personal ones thus are central for the formation and
maintenance of societies and social institutions, and in this context
collective action problems typically occur (norm-based social
institutions are supposed to help to solve them). There is of course
much more to social life, but basically the above considerations,
when spelled out, should cover the most central aspects. As a result
we get a conceptual framework of public, output-related social
concepts which will include also notions related to social positions,
social norms, and social power over and above explicit action
notions.
Roughly in this way we then get the needed "output specifications"
(understood in the above broad sense), and then also the scope of DAI
on a general level becomes determined. As to the question of how to
theorize about these matters and how ultimately to create a theory
satisfying those output specifications, space does not allow me to
say much here. Let me note, however, that here also philosophy has a
role to play: it can be assigned the task to create a broad
conceptual framework of concepts and basic underlying assumptions for
the study of central aspects of social life. This task - because of
its generality and broad scope - goes beyond what social theorists
can be expected to do. What kinds of elements does such a conceptual
framework contain? Here is a short and incomplete list of notions
that philosophers have studied: joint intention, mutual knowledge,
joint and collective action, joint decision-making, social norms of
various kinds, social institutions, agreement and contract,
communication, bargaining, power, strategic interaction, love and
caring, morality, social values and good society (many of these
concepts have been investigated in Gilbert, 1989, Pettit, 1993, and
Tuomela, 1995). These notions - central for the study of social life
- will have to be ultimately taken into account also by DAI.
I have claimed thus that philosophers ought to create relevant social
conceptual frameworks and show DAI in broad terms where to go, what
kind of problems to undertake to solve or at least indicate in what
kind of conceptual setting or settings feasibly to work. (It is of
course clearly the task of DAI rather than philosophy to determine
what its specific research problems ought to be - that depends on the
internal development in DAI and in its neighbor sciences.) Let me
idealize the situation somewhat and make the following slightly
speculative suggestion in terms of the software-hardware distinction:
Think of the (or a) public social framework as a kind of software in
terms of which to theorize, while the hardware will consist of the
agents "realizing" that software and "computing" the programs stated
in terms of that software. The present software-hardware distinction
is not quite clear-cut: While idiosynchratic psychological states and
processes clearly would belong to the hardware side possibly also
some non-general psychological information should be put on the
social software side in a publicly codified form (e.g. in the form
psychological types - altruist, free-rider, dominant).
To continue with the tasks of philosophy, it is also expected to
sketch the viable philosophical options (or at least arguemnts
pro et con ) and investigate their features. For
instance, in the case of the mind-body problem it will specify the
various versions or monism, materialism (or physicalism) and dualism
and discuss their features. (Examples of some recently much discussed
mind-body views are identity theory, token physicalism, supervenience
physicalism, and functionalism.) Each DAI-researcher is committed to,
and relies on, some philosophical background assumptions - whether he
is aware of it or not. The task of philosophy is also to show how DAI
connects to other sciences and disciplines such as the brain
sciences.
Given the above remarks it should be clear that the social conceptual
framework that DAI will ultimately need is quite broad. Thus, for
instance, the Belief-Desire-Intention (BDI) architecture that has
been employed in the study of both single-agent and multi-agent
action will have to be complemented by other social concepts, as
indicated. Over and above helping in the creation of conceptual
frameworks and in the critical investigation of underlying
assumptions and arguments philosophy can also help DAI in more
down-to-earth questions related to actual everyday research. Thus
philosophy can contribute to DAI in the research problems related to
logic, in the analysis of specific notions and topics, questions of
rationality, and in making choices concerning metaphysical options
(e.g. formal ontology, limits of DAI). Analytic philosophers are also
good at inventing substantive, unartificial examples - especially
counterexamples to specific theses.
As researchers in AI know, they owe much of their logical and exact
tools to philosophy. Especially, modal logic, which is so important
in AI today, was first developed by philosophers. Possibly most of
the research in modal logic is now conducted by researchers within
AI, but also philosophers continue to work in that field. I will not
present evidence for this - one only needs to open a textbook in AI
to see the relevance of the work by philosopher-logicians.
Recent developments within the philosophy of mind and the philosophy
of action have helped the concept formation in AI and in part
vice versa (cf. the frequent references to books such as
Bratman, 1987 and the work on belief-kinematics, and - in the other
direction -the influence of AI on the philosophy of mind and even
philosophy of science). As for DAI and multi-agent theory, one can
find traces of influence - and references - in DAI literature to
philosophers' writings, although it must be said that even within
philosophy the questions related to the theory of social action were
not seriously researched before the 1980's. Some relevant pieces of
philosophical work related to joint intention and joint action,
cooperation, and communication that also some DAI-researchers have
noticed are the papers by Bratman (1992, 1993), Grice (1957), Searle
(1990), and Tuomela (1991, 1993a) as well as Tuomela and Miller
(1988). Other relevant pieces of logical and philosophical work are
the works by Belnap (1991), Bratman (1994), Cohen and Levesque
(1991), Gilbert (1989), Tuomela and Sandu (1994), Sandu and Tuomela
(1994), Tuomela (1984, 1993b, 1995). (Relevant works by
DAI-researchers will be referred to and discussed later in this
paper.) As to questions of rationality and metaphysics - mentioned
above - there has not yet been much interaction between philosophy
and DAI. It is easy to predict, however, that questions of individual
and collective rationality - and rationalization - will have to be
tackled also by DAI-researchers in the near future when the study of
robot communities really gets on its way. Then also all kinds of
questions related to the metaphysics of the social realm will have to
be confronted (e.g., the reality of groups and social
structures).
II JOINT INTENTION: THE ANALYSANDUM AND CONDITIONS FOR ANALYSIS
To show that there is philosophical research very close to research
in DAI, I will in the rest of this paper discuss the central
multi-agent notion of joint intention (and to a lesser extent that of
joint action). The material I will present is rather concise, and in
many places I cannot use space for presenting arguments but refer the
reader to other writings. While I will concentrate on my own research
below, some representative examples of other theoreticians' work will
also be considered towards the end of the paper. The purpose of the
discussion in the rest of the paper is nevertheless not to give a
comprehensive survey. Rather it is to show in a detailed fashion how
philosophers and DAI researchers can work hand in hand in this kind
of context.
The theoretical view I will present to you has as its intuitive
analysandum the following situation. Suppose one of us comes up with
the idea of cleaning our backyard. He may express this idea
colloquially by saying "Let's clean our backyard! Are you with me?"
The others may answer "Yes" or "A good idea!". Suppose there are
three persons involved all of whom in this sense accept the idea of
cleaning the backyard. We can say that what they accept is a plan for
doing something together. Another way of putting this matter is that
they have made an agreement to clean the backyard. The intuitive
analysandum here is, anyway, that of accepting - separately or, in
some cases, jointly - a plan to do something together (cf. "Here is a
plan for joint action. Who accepts it"?).
In my analytic terminology we can say the following about the
analysandum. The participants of a joint action to be intentionally
performed have publicly accepted a plan for performing that action
(or at least a plan somehow implying the performance of that action).
The joint action thus is plan-based, and the participants' having and
endorsing that plan involves their joint intention to perform the
action in question. In my analysis I will accordingly say that the
participants conatively endorse a plan involving or entailing the
joint-intention expression "We will do X together", X being a joint
action. (When applied to a certain participant I will say that he has
or shares the "group-intention" to do X; what will be called a
"we-intention" is a special kind of group-intention.) The endorsement
of the plan must be public and in standard cases it must also be
communicated to the other participants. The acceptance of the plan by
the participants can take place purely individually - cf. "This is
the plan. Who accepts it?". The acceptance of the plan, however,
commits the participating persons to action - this is commitment in a
strong interpersonal sense concerned both with the joint action and
the participants' parts or shares, with responsibility to take part
towards the other participants.
I have elsewhere (in Tuomela, 1995) argued that joint intentions are
central in several respects. As I will below be mainly concerned with
analysing the notion of joint intention, it is useful to present a
list of features that indicate the centrality of joint intentions and
also serve as a kind of criteria of adequacy that any analysis of
joint intention should respect. Consider thus the following list
(also cf. Bratman, 1992):
1) Joint intentions figure centrally in participating agents'
(social) practical reasoning and resulting joint action. Thus:
a) Joint intentions - in analogy with single-agent intentions -
present problems and restrict available action alternatives. Forming
a joint intention to go swimming excludes other alternatives and
present problems concerning the means to perform the joint action in
question.
b) Joint intentions serve to initiate, guide, and monitor joint
action, creating order - especially interpersonal coordination - in
social life. Furthermore, there cannot be intentional joint action
without joint intention (at least joint "action-intention").
c) Joint intentions help to connect the group-level with the personal
level. Thus it can be argued that a group's intentional action - e.g.
a business company's buying another company - must be based on the
joint intentions of some authority-possessing members of the
group.
d) Joint intentions have a normative impact on the agents' thinking
and acting. (Recall my remarks on joint commitment.)
2) The concept of joint intention is central both in philosophy of
social science and in theorizing about the social world (and thus it
occurs in some sense in various theorists' heads, so to speak).
Thus:
a) The concept of joint intention (a person's we-intention) is needed
for a characterization of the social "I" (viz. "I" as "one of us").
It will argued below that joint intentions - which are necessary
ingredients of intentional joint action - are irreducible to personal
intentions and other personal notions.
b) The concept of joint intention helps to conceptualize the conflict
between the group-level and personal level interests - cf. collective
action dilemmas - and is central for the analysis of group-phenomena
such as actions performed by groups in terms of what group members
think and do;
c) The notion of joint intention (and related jointness-notions) can
be and should be important in theory-formation in the social
sciences. The most basic argument for this is simply that people have
joint intentions in many central social situations and that therefore
social theory - and also DAI - must deal with the concept of joint
intention.
d) The notion of a social group, in a refined sense, is basically an
"authority-involving" system for arriving at joint intentions
involving group commitments (see Tuomela, 1995, for discussion).
As said, the above features also qualify as a kind of criteria of
adequacy for any account of joint intention. (That criteria 1)a)-d)
will be fulfilled by the account to be offered should be rather clear
from my presentation in the next section; as for 2)a)-d) I refer the
reader to Tuomela, 1995.)
III AN ANALYSIS OF JOINT INTENTION
Suppose that you and I have the joint intention to clean our backyard
together, as expressed by the conative sentence "We will clean our
backyard together". The content of the intention here involves our
performing something together and the pronoun 'we' of course refers
to our group. A joint intention (expressible by a sentence of the
form "We will do X"") is something that several agents have among
themselves.
In contrast to wants, wishes, and hopes, and many other mental
states, intentions are, in the last analysis, necessarily related to
one's own action. Accordingly, when an agent is said to take part in
a joint intention, what he ultimately will have to intend is that he
by his own action, by his part or share, contribute to the joint
action. A fully intentional joint action X (cf. jointly carrying a
table, singing a song) must arguably be based on the participants'
relevant joint intention (normally the intention to do X), and we
should accordingly require that each participant have the intention
to do his part of X. This kind of intention is just an intention that
can be called a we-intention.
Intentions being in part cognitive, an agent cannot intend to perform
an action unless he believes that this action is possible or at least
realizable with some nonnegligible probability. With double analogy,
an agent cannot we-intend unless he believes not only that he can
perform his part of X but also that he together with his fellow
participants can perform the joint action in question: The jointly
intending agents must believe that the "joint action opportunities"
for an intentional performance of X are (or will be) there. Yet
another property of a we-intention is that in each participant's view
it must be mutually believed by the participants that the
presuppositions for an intentional performance of X hold. Even more
must be said, if my plan-view of intention is right: The agents must
believe that they have accepted a plan for joint action, and at least
in standard cases this must be just the we-intended joint action.
What this involves is that the concept of we-intention is
conceptually dependent on a preanalytic notion of joint intention.
Thus we-intentions cannot be analyzed in terms of personal intentions
and mutual beliefs. It can be debated whether the dependence of
we-intention on joint intention is of the nature of conceptual
presupposition or whether it should be built into the concept of
we-intention. If we accept the latter view we must accept that it is
a conceptual truth in the case of a we-intending agent A that he
we-intends to do X in accordance with and because of the
participants' joint intention to do X. This can be argued for by
reference to the satisfaction conditions of the we-intention: Unless
A performed his part of X because of the participants' plan to do X
he surely would not have satisfied his we-intention.
Actually joint intentions must be taken to include also standing
intentions. We-intentions directly generate action and do not include
all standing joint intentions, e.g. those concerned with future
actions about whose performance ("joint action opportunities") there
is no definite belief yet. Group-intentions in this sense can be
regarded as precisely those intentions which are expressible by
locutions of the form "We will do X" (where 'will' is conatively
used). Arguably, such group-intentions are either we-intentions or
dispositions to we-intend, and I have argued that joint intentions
can be regarded basically as mutually shared we-intentions (or at
least dispositions to we-intend) about whose existence the
participants have mutual knowledge (or belief). (See Tuomela, 1991,
and especially 1995, Chapter 3.)
In contrast to joint desires and wants, we can say the following
about joint intentions (shared we-intentions): Based on their various
personal and, especially, joint desires and mutual and other beliefs,
the agents make up their minds - jointly or separately - and thereby
form an intention (or plan, as we may say) to act jointly, attempting
to act rationally in a coordinated way so as to fulfil the already
formed plan. Joint intentions, in contrast to joint wants and
desires, can be regarded as joint commitments to action, viz. the
participants' interdependent commitments to perform their parts of
the joint action and their responsibility for the total joint action
getting performed. (We can also say more generally that the
participants are jointly committed to reach a joint goal - such as
X's having been performed jointly by them - and their plan to achieve
this joint goal, as reflected in their part-performances.) Such joint
commitments are appropriately persistent and, especially, are not
consummated before the agents have jointly achieved what they
we-intend (or achieved consensus about the unachievability of the
intended goal). However, as in the case of mere personal intentions,
the agents can change their mind, and thus joint commitments are not
irrevocable. It is nevertheless a rationality condition that the
participants preserve their we-intentions as long as the others do or
else at least inform the others about their changes of mind in order
to become released from their commitment to participate in the
intended joint action.
Let us now think of some agents' having accepted "We will jointly
clean our backyard" and thus agreed to act jointly. This acceptance
must be conative - they must be disposed to contribute to the joint
task and also be normatively committed to that task. Joint intentions
are formed in such a process of agreement-making (not all
agreement-making needs to result in a joint intention). In
joint-intention formation each agent also accepts for himself: "I
ought to participate in our doing X together". This acceptance means
not only that the agent (at least dimly) recognizes the existence of
the agreement ("plan") to perform X but also commits himself to
performing X together with the others.
The agreement-making that takes place in the context of the formation
of a joint intention can take place fully explicitly or only
implicitly. Agreement-making - be it explicit or not - is an
essentially intentional activity and also a communication-based
public activity. Mistakes and errors about the contents of agreements
can of course occur, but in the standard cases the participants will
share the correct mutual belief not only about that they have made an
agreement but also about its content. According to the agreement-view
of joint intention and joint action every full-blown or "proper"
intentional joint action and - accordingly joint intention - must be
based on an agreement by the participants. However, this is only the
core idea. There are cases of a) pre-agreement and b) institutional
cases in which no agreement-making concerning the particular action
in question is needed. (I shall not here discuss these issues further
- see Tuomela, 1995, Chapter 2.)
Let us consider the central notion of we-intention in some more
detail. We-intentions are action-generating joint intentions that
agents have in situations of joint action, e.g., when they intend to
carry a table jointly. The content of a we-intention can (with some
reservations) be taken to be "to do X jointly" (or, to emphasize its
"we-character", something like "we to do X jointly"). A we-intention
involves the intention to perform one's part of the joint action. I
will now present a detailed summary analysis of the notion of
we-intention (cf. Tuomela, 1995). We consider a member Ai of a
collective G ("we" for Ai) assumed to consist of A1,...,Am, with i
=1,...,m (and allowing the word 'agreement' to refer to merely
believed agreement):
(WI ) A member Ai of a collective G
we-intends to do X if and only if, based on the
(explicit or implicit) agreement to perform X jointly made by the
agents A1,...,Ai,...,Am,
(i) Ai intends to do his (agreement-based) part of X (as his part of
X);
(ii) Ai has a belief to the effect that the joint action
opportunities for an intentional performance of X will obtain (or at
least probably will obtain), especially that a right number of the
full-fledged and adequately informed members of G, as required for
the performance of X, will (or at least probably will) do their parts
of X, which will under normal conditions result in an intentional
joint performance of X by the participants;
(iii) Ai believes that there is (or will be) a mutual belief among
the participating members of G (or at least among those participants
who do their parts of X intentionally as their parts of X there is or
will be a mutual belief) to the effect that the joint action
opportunities for an intentional performance of X will obtain (or at
least probably will obtain);
(iv) (i) in part because of (ii) and (iii).
In this analysis clause (i) is the most central one; (ii) and (iii)
can be regarded as presupposition-beliefs, whose effectiveness is
guaranteed by (iv).
We actually need a general notion of joint intention (or
group-intention), viz. a notion which covers not only
action-generating but also standing joint intentions.
Put somewhat differently, we need a characterization of the notion of
joint intention satisfying the truth conditions of the conatively
used, intention-expressing sentence "We will do X", where X denotes
an arbitrary joint action type. While it is rather obvious that
we-intentions in the sense of our (WI ) make this
sentence schema true, there are also other truthmakers. I have argued
in Tuomela (1991) that conditional we-intentions with conditions
believed by the agent to be satisfied also qualify, and so also do
dispositions to we-intend in general. More exactly, I defend
essentially the following analysis:
(GI ) "We will do X" is true of A (relative to A's group
G) if and only if, based on the (explicit or implicit) agreement by
the members of A's group G to perform X jointly,
1) A we-intends to do X (in the sense of (WI )); or
2) A has formed a standing group-intention to do X, which is a
disposition to we-intend to do X (provided X has a
part-division).
Group-intentions can now be regarded as either we-intentions or
dispositions to we-intend. On the basis of our earlier discussion we
can now give our analysis of the notion of joint intention:
(JI ) Agents A1,...,Ai,...,Am have the joint
intention to perform a joint action X if and only if
a) these agents have the group-intention to perform X; and
b) there is a mutual belief among them to the effect that a).
The upshot of our present analysis is this. Joint
intentions are intentions that several agents among themselves
have, and they are expressible by an intention-expression of the form
"We will do X" endorsed by these agents. When two or more agents have
the joint intention to do X each of these agents also accepts the
intention-expression "We will do X" and essentially because of that
they can be said to have the group-intention to do X.
Furthermore, when the joint action X has a - preassigned or presently
agreed-on - part-structure, the an agent's group-intention to do X is
either his we-intention to do X or he is disposed
to we-intend X. Finally, an agent's we-intention to do X
amounts to his intention to do his part of X (as his
part of X) and some relevant beliefs presupposed in this situation.
These intention concepts form a family of concepts which cannot be
reduced away (e.g. in terms of personal intentions and mutual
beliefs). The irreducibility of the notion of joint intention is also
indicated by the fact that a person's we-intention contains implicit
reference to a preanalytic notion of joint intention: A person cannot
we-intend to do X without we-intending in accordance with and because
of the agents' joint intention (plan) to do X together.
Up to now I have used both the agreement-terminology and the
plan-terminology to speak about joint intentions. Each of the notions
of agreement-making and plan-acceptance would suffice as my central
analytic concepts. In fact, as I use these notions, they are
equivalent in a clear and obvious sense. Thus, the following thesis,
or at least its core idea, can be argued to be acceptable:
(JIP ) Some agents (say A1,...,Ai,...,Am) have formed
the (agreement-based) joint intention to perform X if and only if
each of them a) has accepted a plan to perform X jointly, b) has
communicated this acceptance to the others, and c) because of a) and
b) it is a true mutual belief among A1,...,Ai,...,Am that they are
jointly committed to performing X and that there is or will be a part
or share (requiring at least potential contribution) of X for each
agent to perform that he accordingly is or will be committed to
performing.
In other words, while the concept of a joint intention is different
from the notion of accepting and endorsing a plan to act jointly,
still having a joint intention amounts to accepting and endorsing a
plan, provided the agents' have - in the simple core cases at least -
communicated their acceptance to each other and have become jointly
committed to carrying it out (see Tuomela, 1995, Chapter 3 for a
discussion).
IV OTHER APPROACHES TO JOINT INTENTION
The study of the notions of joint action and joint intention has not
been a popular topic in philosophy. While there is a huge literature
on action and intention and related notions in the single-agent case
there is relatively little systematic work available concerning the
corresponding joint notions. I will below briefly consider all the
philosphical approaches that are sufficiently precise to be compared
with my above account. These are Bratman's and Searle's accounts.
From the approaches published by researchers in DAI I will comment on
Cohen's and Levesque's account and briefly also on a paper by Rao,
Georgeff, and Sonenberg.
Recently Michael Bratman - who is well-known in AI for his work on
single-agent intention - has begun to investigate multi-agent
notions. In a recent paper Bratman (1993) analyzes shared
intentions in terms of the locutions "We intend to J", where J
is a joint action. His final analysis is this (for the two-person
case):
We intend to J if and only if
1) a) I intend that we J and b) you intend that we J;
2) a) I intend that we J in accordance with and because of 1)a),
1)b), and meshing subplans of 1)a) and 1)b); you intend that we J in
accordance with and because of 1)a), 1)b), and meshing subplans of
1)a) and 1)b).
3) 1) and 2) are common knowledge between us.
I shall not discuss Bratman's notions in detail here but only show
that it is not only conceptually different but also classifies cases
of shared and joint intention differently from my account. Basically
his approach operates with mere personal intentions which concern a
joint action. The intentions are interdependent (clause 2)) and
mutual knowledge about the agents' intentions is assumed (clause 3)).
This conceptually contrasts with my plan-conception which regards the
concept of joint intention irreducible to personal intentions and
mutual knowledge.
The following argument shows that Bratman's shared intentions need
not always be joint intentions in my sense. According to him the
following example satisfies his analysis: "You and I arrive at a
public basketball court and simply begin, without bothering to assure
each other of our intentions, to take turns shooting." According to
me, this is best analyzed as a case of mere shared intention based on
what I have called a "proper" social norm (one based on mere mutual
belief rather than agreement-making), not as a case involving a
proper joint intention. Both agents shared a social norm and intend
to obey it and there is mutual knowledge about this: this gives the
intention to shoot, and Bratman's above conditions for shared
intentions are fullfilled, or so Bratman at least assumes. The
fulfillment of his clauses does not entail the existence even of an
implicit agreement and hence not of a proper joint intention. His
analysis accepts the case of shared and suitably interconnected
personal intentions - which may be based merely on norm-obeying as
above. The basketball example also is a case without strong
interpersonal obligations - but there is no joint intention in my
plan-sense either. So the case is compatible with my analysis and
also serves to show that there are cases of shared intention in
Bratman's sense which fail to be cases of joint intention in my
sense. I claim that my analysis is more informative as an analysis of
joint intention in that it discriminates better between such diverse
examples as personal norm-obeying and "true joining of wills" which
takes place in in the case of agreement-based joint intention.
However, Bratman's notion of shared intention may be of interest in
its own right.1
Let me next consider the account by the philosopher John Searle
(1990).2 He adopts a view of collective intentions which is rather
close to my view (which I have presented and elaborated - with some
changes and corrections - in a number of publications since the late
1970's). His view of the content of we-intention resembles what I
have said about the satisfaction conditions of we-intention.
According to him the content (satisfaction conditions) of a
we-intention is means-relative: Ai intends to perform joint action X
by means of his single-agent action Xi. But this idea holds true in
my account as well, for the parts Xj, j = 1,...,m, are means to the
m-agent joint action X. Searle gives the following formula for the
satisfaction conditions of a we-intention (intention-in-action,
i.a.), B being the joint action of mixing a sauce (p. 414):
i.a. collective B by means of singular A (this i.a. causes: A
stirred, causes: B mixed).
This is not quite right, because it ignores that a participant's
part-performance is dependent on the other participants' part
performances. But corrected for this, Searle's account - when
corrected also for lack of the resources of "conceptual action
generation"- more or less amounts to my view. We then get basically
this:
Ai has a we-intention to perform a joint action X by means of his
performing his part Xi (as his part of X), presuming that the other
participants perform their parts of X; this we-intention has as its
conditions of satisfaction that this very we-intention (causally
and/or conceptually) generates Ai's performing his part Xi, which,
presuming the other participants' performances of their parts,
(causally and/or conceptually) generates an intentional performance
of X by the participants.
Let me now briefly consider how researchers in DAI have analyzed
joint intention. Actually the only published work I have seen comes
from two sources. First, Cohen and Levesque and their co-workers have
presented a detailed full-blown theory of intention and action both
for the single-agent and the multi-agent case. Their theory is surely
a very fine achievement, although I do have some criticisms
concerning its details. Nobody working on joint intention within DAI
or in any other field can afford not to study the papers of these
authors. I shall below sketch in informal terms - used by Cohen and
Levesque themselves - some basic features of their complicated theory
and I will also present some critical remarks against it.
In Cohen and Levesque (1991) the following definition of joint
intention is given:
Definition. A team of agents jointly
intends , relative to some escape condition, to do an action,
iff the members have a joint persistent goal relative to that
condition of their having done that action and, moreover, having done
it mutually believing throughout that they were doing it.
The notion of a joint persistent goal used here is analyzed as
follows:
Definition : A team of agents have a joint
persistent goal relative to q to achieve p just in case
1) they mutually believe that p is currently false;
2) they mutually know they all want p to eventually be true;
3) it is true (and mutual knowledge) that until they come to mutually
believe either that p is true, that p will never be true, or that q
is false, they will continue to mutually believe that they each have
p as a weak achievement goal relative to q and with respect to the
team.
The notion of a weak achievement goal is this:
Definition : An agent has a weak achievement
goal relative to q and with respect to a team to bring about p
if either or these conditions holds:
i) The agent has a normal achievement goal to bring about p, that is,
the agent does not yet believe that p is true and has p eventually
being true as a goal.
ii) The agent believes that p is true, will never be true, or is
irrelevant (that is q is false), but has as a goal that the status of
p be mutually believed by all the team members.
The conditions about the existence of joint action and joint
intention opportunities mentioned above seem to be correct. Thus,
surely when having a joint goal or intention that p, the participants
are assumed not to mutually believe that p or that necessarily -p or
that -q. (These conditions are also covered in my account, not as
conditions in the analysis itself but as presuppositions of
analysis.) What is especially valuable in these authors' analysis is
that they show what role divergent "private" beliefs about the
situation had by the participants may play here. For instance, such a
private belief when having the content that the joint action after
all faces insurmountable obstacles may lead a participant to switch
his original goal of performing his part of the joint action into the
goal - or, as we should rather say, obligation - of creating the
mutual belief among the participants that the goal of the joint
action is not attainable.
However, as seen from the above summary, this account does not refer
to either explicit or implicit agreement (not even to believed
agreement); nor is obligation or any other normative notion involved
in the analysans; finally, communication is not "officially" required
in the Cohen-Levesque theory. In my account presented in Section V
these features, in contrast, play a central role, and suggest to me
that my analysandum may be stronger than in these authors' account.
Furthermore, in their theory, joint intention and joint commitment
are basically analyzed in terms of goals (chosen, consistent desires)
and mutual (and other) beliefs. This analysis problematically reduces
away intentions - unless their notion of choice here is a necessarily
intentional notion. Their theory also involves the possibility that
noncommunicating robots or any similar mechanical devices having
beliefs or belief-like states but lacking normative conceptual
resources could have joint intentions and commitments. From the point
of view of our theory this is implausible at least if full-blown
joint intention is meant. Mutual belief surely is necessary in this
context, but the normative notion of agreement (at least belief in
agreement) is also needed. It seems that cases of mere norm-following
with rigid normative constraints for the beginning and end of the
instances of norm-following would qualify as joint intentions in the
present authors' sense (cf. the Finnish ritual of lighting candles in
the evening of the Independence Day).3 From a normative point of
view, it can be said that the analysans in the account of Levesque et
al., because involving no normative notions, seems to me not to be
fully able to account for the notion of joint commitment, no matter
how well it may capture the descriptive aspects of commitment.
I have argued above (and in other works) that conceptually the
notions joint intention, joint commitment, and joint action can only
be based on the normative (viz. joint-commitment entailing) notion of
agreement (or believed agreement), implicit or explicit, and that, on
factual grounds, recurrently successful joint action would be a
surprising phenomenon in cases of several agents trying to act
jointly merely on mutual belief, without agreement, about what they
and the others should and will do in a joint project. This suggests
to me that the Cohen-Levesque approach, no matter how detailed and
precise, is not fully adequate.
In a recent paper the DAI-researchers Rao, Georgeff, and Sonenberg
(1992) present a technical account of joint intention. While in the
Cohen-Levesque logical semantics possible worlds are time-lines the
present authors take possible worlds to be trees. This gives more
logical flexibility but also complicates the analysis. However, I
cannot here go into technical questions. As to joint intention in the
present approach it is a primitive notion which is connected to plans
and personal attitudes (intentions, goals, and beliefs).4 While these
authors' work surely is interesting from a logical point of view, it
- perhaps because of its preliminary nature - adds relatively little
to the conceptual features of joint intention.
V CONCLUSION
I hope the above presentation has shown that work on jointness
notions is in full swing both in philosophy and in DAI and also that
there is genuine interaction between the research done in these two
different fields. One would expect that in the near future - possibly
in a few years - most of the central social notions mentioned in
Section I of this paper will a topic of interest and - hopefully - of
actual work within DAI. There is one good reason for this
expectation: As work on multi-agent notions gets further it simply
has to - due to conceptual-theoretical reasons - deal with a number
of central, intertwined notions that so far have not been studied by
DAI-researchers. The BDI-paradigm will have to be expanded to deal
with - for instance - agreement, joint decision, social power,
communication, and various institutional social concepts.
Notes
1) In Tuomela (1994) I argue that Bratman's analysis - which is not
based on the plan-sense of joint intention - is too weak even as an
analysis of shared intention. The basic point of the argument is that
the agent's intentions in clause 1) of his analysis must be
responsive to the mutual knowledge of clause 3). But when corrected
for this, Bratman's analysis becomes formally equivalent to the
analysis of the notion of an intended collective goal that I advocate
in the mentioned book. This notion is wider than the plan-based
notion of joint intention and is also reducible to individual
intentions and beliefs. It has important applications to theory of
social action.
2) Searle (1990) presents a criticism of the account of we-intention
presented in Tuomela and Miller (1988). I argue in Tuomela (1995), in
a long footnote to Chapter 3, that Searle has badly misunderstood our
position and that therefore his criticism has very little to do with
what our view is. He believes that our position is that joint
intentions can be reductively defined in terms of personal intentions
and mutual beliefs. As seen above, this is not my view. I take
Searle's criticism against that reductive position to be right.
3) Castelfranchi et al. (1993) give another kind of counterexample.
Two researchers (or, if you prefer, research teams) have the
intention to find a vaccine against AIDS. They argue that this shared
intention amounts to a joint intention in the sense of Cohen and
Levesque. However, as the researchers compete with each other their
shared intention cannot be a joint intention.
4) Rao et al. give their definition on joint intention on p. 68. Let
me try to give its basic content verbally: The members of a
collection of agents jointly intend that something p if and only if
everyone in the collection intends that p and there is a mutual
belief among them that they jointly intend that p. Thus we can see
that the notion of joint intention occurs irreducibly in the content
of the agents' mutual belief. This results seems to me to contradict
these authors' statement on p. 60 according to which "all joint
propositional attitudes are defined in terms of the individual
propositional attitudes".
Rao et al. regard my (and my coauthor Miller's) work as reductive:
"Tuomela and Miller [1988] and Grosz and Sidner [1990] regard a joint
intention as definable in terms of the conjunction of individual
intentions together with mutual beliefs about the intentions of other
individuals". As seen, this claim does not hold true of our approach
while it may apply to the view of Grosz and Sidner.
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