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\title {Origins of Mind: Lecture Notes \\ Joint Action}

\maketitle

# Joint Action

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\def \isubtitle {Joint Action}

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What is joint action?
One way to get an initial, pre-theoretical fix on the notion is to adapt an idea from David Pears and think about contrasts between joint actions and parallel but merely individual actions.

\section{Joint Action vs Parallel but Merely Individual Action}

\section{Joint Action vs Parallel but Merely Individual Action}
Two sisters cycling to school together are engaged in a joint action,
whereas two strangers who happened to be cycling the same route side-by-side would not be (compare \citealp{gilbert_walking_1990}).
Two sisters cycling to school together exercise shared agency whereas two strangers who happened to be cycling the same route side-by-side do not (compare \citealp{gilbert_walking_1990}).
Similarly, when members of a flash mob in the Central Cafe respond to a pre-arranged cue by noisily opening their newspapers, they perform a joint action.
But when someone not part of the mob just happens to noisily open her newspaper in response to the same cue, her action is not part of any joint action.%
\footnote{
See \citet{Searle:1990em}; in his example park visitors simultaneously run to a shelter, in once case as part of dancing together and in another case because of a storm.
Compare \citet{Pears:1971fk} who uses contrast cases to argue that whether something is an ordinary, individual action depends on its antecedents.
}
When members of a flash mob respond to a pre-arranged cue by noisily opening their newspapers, they exercise shared agency. But when others happen to noisily open their newspapers in response to the same cue, they do not \citep[compare][]{Searle:1990em}.
These and other contrast cases invite the question,
\textbf{What distinguishes joint actions from parallel but individual actions?}

\textbf{Question} What distinguishes joint actions from parallel but individual actions?
The first contrast case shows that the difference can’t be just a matter of coordination because people who merely happen to be cycling side-by-side also need to coordinate their actions in order to avoid colliding.
Note also that in both cases each individual's cycling is intentional, so our intentionally cycling together cannot be only a matter of our each intentionally cycling.
The second contrast case shows that the difference can’t just be that the resulting actions have a common effect because merely parallel actions can have common effects too.

At this point it is natural to appeal to intention.
If we are performing actions of some type phi,
then perhaps for our doing this to be a joint action is just for us to be doing this
because we each intend that we, you and I, phi together.
So in the case of the cycling sisters, each would intend that they, the two sisters cycle to school together.
I'm going to come back to this idea so it's helpful to give it a label.
Let's call it the Simple Account of intentional joint action.

each sister intends that
they, the sisters, cycle to school together?

Does the appeal to togetherness make this circular? Not as long as we understand 'together' only in the sense in which the three legs of a tripod support the flask \emph{together}.
So we have to understand the intention as concerning an event type that could be a joint action but might also involve merely parallel actions.

\textbf{Simple Account}
(Intentional) joint action occurs when there is an act-type, φ, such that each of several agents intends that they, these agents, φ.
This certainly distinguishes the cases on your left from those on your right.
But we can see that the Simple Account is too simple as it stands by adapting an example from Gilbert and Bratman ...
Contrast two friends walking together in the ordinary way,
which is a paradigm case of collective agency,
with a situation where two gangsters walk together but each is forcing the other.
It works like this: Gangster 1 pulls a gun on Gangster 2 and says: “let’s walk”
But Gangster 2 does the same thing to Gangster 1 simultaneously.
This is walking together in the Tarrantino sense,
and clearly not a case of joint action.
At least it’s not joint action unless the central event of of Reservoir Dogs is also a case of joint action.
Since in this case there is something which all the agents involved intend, it seems that our being involved in a joint action can't be a matter only of there being something such that we each intend that we do it together.
The apparent difficulty of giving sufficient for shared intention has motivated some philosophers to invoke conceptually novel ingredients in explaining intentional joint action, novel ingredients such as plural subjects of attitudes \citep[e.g.][]{schmid:2009_plural_bk}, special attitudes like what Searle calls a we-intention \citep[e.g.][]{Searle:1990em}, or novel kinds of reasoning, team reasoning \citep{Gold:2007zd}.
One way to oppose the claim that any such special ingredients are needed is to argue that sufficient conditions for intentional joint action can be given without them. This is Bratman's project. He wants to show that our question, What distinguishes joint action from parallel but merely individual action, can be answered without appeal to any conceptually novel ingredients.

\section{Bratman on Shared Intention}

\section{Bratman on Shared Intention}
Bratman’s brilliant idea for avoiding this sort of problem is to suggest that we don’t just each intend the action but rather we each intend to act by way of the other's intentions.
We can put this by saying that our intentions must interlock: mine specify yours and yours mind.
Now this appeal to interlocking intentions enables Bratman to avoid counterexamples like the Tarantino walkers; if I intend that we walk by way of your intention that we walk, I suppose can't rationally also point a gun at you and coerce you to walk.

‘each agent does not just intend that the group perform the […] joint action.

‘Rather, each agent intends as well that the group perform this joint action in accordance with subplans (of the intentions in favor of the joint action) that mesh’

(Bratman 1992: 332)

each agent does not just intend that the group perform the […] joint action. Rather, each agent intends as well that the group perform this joint action in accordance with subplans (of the intentions in favor of the joint action) that mesh' \citep[p.\ 332]{Bratman:1992mi}.
Our plans are \emph{interconnected} just if facts about your plans feature in mine and conversely.
‘shared intentional [i.e.\ collective] agency consists, at bottom, in interconnected planning agency of the participants’ \citep{Bratman:2011fk}.
In making this idea more precise, Bratman proposes sufficient conditions for us to have a shared intention that we J ... ... the idea is then that an intentional joint action is an action that is appropriately related to a shared intention.

We have a shared intention that we J if

‘1. (a) I intend that we J and (b) you intend that we J

‘2. I intend that we J in accordance with and because of la, lb, and meshing subplans of la and lb; you intend [likewise] …

‘3. 1 and 2 are common knowledge between us’

(Bratman 1993: View 4)

\begin{minipage}{\columnwidth}
\emph{Bratman’s claim}. For you and I to have a collective/shared intention that we J it is sufficient that:
\begin{enumerate}[label=({\arabic*}),itemsep=0pt,topsep=0pt]
\item (a) I intend that we J and (b) you intend that we J;
\item I intend that we J in accordance with and because of la, lb, and meshing subplans of la and lb; you intend that we J in accordance with and because of la, lb, and meshing subplans of la and lb;
\item 1 and 2 are common knowledge between us' \citep[View 4]{Bratman:1993je}
\end{enumerate}
\end{minipage}
Note that these conditions are sufficient but not plausibly necessary. If sharing a smile is a joint action, and if human infants in their first year of life are incapable of knowing things about other's knowledge of their intentions about the other's intentions, then to suppose that these conditions were necessary would be to imply that you can't share a smile with an infant. For this sort of reason, some have suggested that Bratman is right about one form of shared agency, but that there are other, less demanding forms of shared agency (Tollefsen, Pacherie). I think Bratman himself is coming around to the view that this might be right. I used to think this too, but recently I’ve come to the view that Bratman is more fundamentally wrong about shared intention.
In this talk I'll give a counterexample to Bratman's claim that these conditions, (1)--(3), are collectively sufficient for shared intention. My aim in doing this is to highlight an ingredient missing from Bratman’s and others’ accounts of shared intention. To anticipate what I'll explain later, Bratman focuses on interlocking intentions and interconnected planning. But in some or all cases it is not sufficient that our intentions interlock: in some cases, intentional joint action requires that we each make a single plan for our own and each other’s actions. This means taking a perspective that allows us to see our actions, yours and mine, as having a kind of practical unity. Or so I aim to show.
But first the counterexample.
In constructing the counterexample I shall exploit a feature of Bratman's account.
Needed to avoid circularity ... so this has to be neutral wrt shared intentionality.
It's also important, for Bratman's account and for my purposes, that I can intend *unilaterally* that we J. That is, I can intend that we J without depending on your also intending that we J.
Use ‘blocking the aisle’ as an example an action that can, but needn’t be, an exercise of shared agency. There are two people ahead of us on the train. They are blocking the aisle. We don’t know whether they are doing this intentionally or accidentally, nor whether their doing this is an exercise of shared agency or not. But still we can decide, in a sufficiently perverse mood, we shall do what they are doing. We can decide this without knowing whether what they are doing is an exercise of shared agency or not. The thing we are deciding to perform is an act-type that is neutral with respect to shared intentionality. Now this has a consequence that will be important for me. It means that one of us can intend that we J unilaterally, that is, irrespective of what the other intends. Suppose I know that you are going to stand where you are standing whatever I or anyone else does. You are rooted to the spot. Then I can decided that we will block the aisle, and I can decide this unilaterally because I know that you will do your part despite having no such intention.
This is the key to the possibility of UNSHARED INTENTION ...

\section{Counterexample to Bratman}

\section{Counterexample to Bratman}
The conditions for unshared intention are just like those for shared intention except that they concern two distinct activities, J1 and J2.
So for you and I to have an unshared intention that we , ...
If it is possible for Bratman's sufficient conditions for shared intention to be met without relevant irrationality or ignorance, then it is likewise possible for these conditions on unshared intention to be met.
\begin{minipage}{\columnwidth}
We have an \emph{unshared intention} that we <J$_1$, J$_2$> where J$_1$$\neq$J$_2$ just if:
\begin{enumerate}[label=({\arabic*$^\prime$}),itemsep=0pt,topsep=0pt]
\item (a) I intend that we J$_1$ and (b) you intend that we J$_2$
\item I intend that we J$_1$ in accordance with and because of la, lb, and meshing subplans of la and lb; you intend that we J$_2$ ...
\item 1 and 2 are common knowledge between us.
\end{enumerate}
\end{minipage}
Our individual subplans concerning our <J$_1$, J$_2$>-ing \emph{mesh} just in case there is some way I could J$_1$ and you could J$_2$ that would not violate either of our subplans but would, rather, involve the successful execution of those subplans.

We have an unshared intention that we <J1, J2> iff

‘1. (a) I intend that we J1 and (b) you intend that we J2

‘2. I intend that we J1in accordance with and because of la, lb, and meshing subplans of la and lb; you intend [likewise] …

‘3. 1 and 2 are common knowledge between us’

We have a shared intention that we J if

‘1. (a) I intend that we J and (b) you intend that we J

‘2. I intend that we J in accordance with and because of la, lb, and meshing subplans of la and lb; you intend [likewise] …

‘3. 1 and 2 are common knowledge between us’

(Bratman 1993: View 4)

Here is an example of two people who have an unshared intention.
Ayesha and Ahmed. They can each tilt the table, but only along one axis.
(Note that Ayesha can unilaterally intend that they, Ayesha and Ahmed, make the ball hit the red square.)

If you think Ayesha and Ahmed are having a bad hair day, you should see Beatrice and Baldric ...
Now explain that Ayesha and Ahmed have an unshared intention, but Beatrice and Baldric have a shared intention.
I claim that Beatrice and Baldric have a shared intention that they J$_1$ only if Ayesha and Ahmed have a shared intention. This claim follows from the similarities of the two cases. The only difference is that Beatrice and Baldric happen to have same task, whereas Ayesha and Ahmed have different tasks. But neither Beatrice nor Baldric makes use of the fact that they have the same task. So if we consider how Beatrice and Baldric's case differs from Ayesha and Ahmed’s, we can see that these differences do not plausibly amount to a difference with respect to shared agency. Shared intention cannot feature in one case but not the other.
This is a bit delicate. I am supposing that Beatrice and Baldric are each making use of the fact that Beatrice intends J1 and of the fact that Baldric intends that J2, but that they are neglecting to make any use of the fact that J1=J2.
So the only difference is that Beatrice and Baldric happen to have same task, whereas Ayesha and Ahmed have different tasks. But neither Beatrice nor Baldric makes use of the fact that they have the same task.
Beatrice does rely on the fact Baldric intends that they J1, of course; but she does not rely on the fact that what Baldric intends is what she intends.

 true? A&A make use of? Ayesha intends J1 ✓ ✓ Ahmed intends J2 ✓ ✓ J1=J2 ✗ ✗

 true? B&B make use of? Beatrice intends J1 ✓ ✓ Baldric intends J2 ✓ ✓ J1=J2 ✓ ✗

Now so far I've said that that BnB have a shared intention only if AnA do: this isn't yet to say that BnB don't have a shared intention.
I do think it's intuitive that they lack a shared intention, however. What could be missing from Beatrice and Baldric's case that makes their actions not an exercise of shared agency? I think one factor is that they don't themselves conceive of their agency as in any relevant way shared, nor of their actions as joint actions. Lots of actions involve others' agency without thereby being exercises of shared agency. From Beatrice and Baldric's points of view, theirs is just such an action. From their point of view, what they are doing no more involves shared agency than do the actions of a golfer whose hole-in-one depends on the hard work of a gardener keeping the grass short and flat.
perhaps I could make it more convincing that it isn't shared agency by filling in some detail about why Beatrice intends that *they*. (Perhaps there's some further rule that Baldric will be punished if they make the ball hit the blue cross and she wants Baldric to be punished so it matters to Beatrice that it's they not just her who makes the ball hit the blue cross, but for reasons that don't point to shared agency.)
So, at least provisionally, we can add Beatrice & Baldric to the right side of our list of cases of parallel but merely individual action.
This is a case where we have interconnected planning but no shared agency.
I'll strengthen the case for denying that BnB have a shared intention later by constructing a contrasting case in which there really is a shared intention.
(I might mention that there are also mundane counterexamples.)

Joint Action

Parallel but Merely Individual Action

Beatrice & Baldric’s making the cross hit the red square

Two sisters cycling together.

Two strangers cycling the same route side-by-side.

Members of a flash mob simultaneously open their newspapers noisily.

Onlookers simultaneously open their newspapers noisily.

So I reject Bratman's claim that Bratman's conditions are sufficient for shared intention.

We have a shared intention that we J if

‘1. (a) I intend that we J and (b) you intend that we J

‘2. I intend that we J in accordance with and because of la, lb, and meshing subplans of la and lb; you intend [likewise] …

‘3. 1 and 2 are common knowledge between us’

(Bratman 1993: View 4)

\section{Parallel Planning}

\section{Parallel Planning}

Parallel Planning

What’s wrong with B&B is that they don’t conceive of their actions as an exercise of shared agency.
What we want is some way to capture the sense that agents engaged in shared agency conceive of their actions as exercises of shared agency, without of course going in a circle by appealing directly to shared agency here.
At this point it’s tempting to appeal to romantic notions of sharing, or to introduce distinctive ingredients like special modes of thought, special ontological constructs or special kinds of reasoning. I want to suggest a way of capturing the agents’ perspective without any such distinctive ingredients. This is where parallel planning comes in. Let me explain ...
A representation or plan is \emph{agent-neutral} if its content does not specify any particular agent or agents; a planning process is agent-neutral if it involves only agent-neutral representations.
Practical vs theoretical reasoning: ‘The mark of practical reasoning is that the thing wanted is \emph{at a distance} from the immediate action, and the immediate action is calculated as a way of getting or doing or securing the thing wanted’ \citep[p.\ 79]{Anscombe:1957ln}. See also \citet[p.\ 1]{millgram:2001_practical}: ‘Practical reasoning is reasoning directed towards action: figuring out what to do, as contrasted with figuring out how the facts stand.’
Some agents each \emph{individually make a plan for all the agents' actions} just if: there is an outcome; each agent individually, without discussion, communication or prior arrangement, plans for that outcome; and each agent’s plan specifies roles for herself and all the other agents.
Our planning is \emph{parallel} just if you and I are each planning actions that I will eventually perform and actions that you will eventually perform, where the resulting plans non-accidentally match.
Two or more plans \emph{match} just if they are the same, or similar enough that the differences don't matter in the following sense. First, for a particular agent's plan, let the \emph{self part} be those steps concerning what will be the agent's own actions and let the \emph{other part} be the other steps. Now consider what would happen if, for a particular agent, the other part of her plan were as nearly identical to the self part (or parts) of the other's plan (or others' plans) as psychologically possible. If the agent's self part would not be significantly different, let us say that any differences between her plan the other's (or others') are not relevant for her. Finally, if for some plans the differences are not relevant for any of the agents, then let us say that the differences don't matter.
The guiding idea behind Bratman's conditions for shared intention is this:
shared agency consists, at bottom, ...

shared intentional agency consists, at bottom, in interconnected planning agency of the participants.’

(Bratman 2011, p. 11)

We have a shared intention that we J if

‘1. (a) I intend that we J and (b) you intend that we J

‘2. I intend that we J in accordance with and because of la, lb, and meshing subplans of la and lb; you intend [likewise] …

‘3. 1 and 2 are common knowledge between us’

(Bratman 1993: View 4)

parallel planning

You plan our actions, yours and mine, and I plan our actions too

Here, interconnected planning is planning where facts about your plans feature in my plans & conversely. What I've just tried to show is that interconnected planning is not sufficient for joint action. Ayesha's and Ahmed's plans are interconnected and so are Beatrice's and Baldric's, but still each sees the other's actions only as opportunities to exploit and constraints to work around. Now my question is what would be sufficient for joint action ...
Suppose you and I are tasked with moving this table through that door.
In doing this, must my plan take into account facts about your intentions as well as about the weight of the table, width of the door &c?
This case has some special features: (i) there is a single most salient route for the table given our objective; (ii) there is a single most salient way of dividing up the roles between us.
I suggest that, in this situation, neither of us needs to form a plan involving the others' intentions.
The situation makes this redundant.
All we have to plan is how two people in our situations should move the table through the door.
To a first approximation, then, what the situation seems to call for is not that our plans are interconnected but rather that we each make a plan for the table-moving action as a whole. This is inspiration for the view that we might arrive at sufficient conditions by reflecting on parallel rather than interconnected planning ...

shared intentional agency consists, at bottom, in interconnected planning agency of the participants.’

(Bratman 2011, p. 11)

parallel planning

You plan my actions as well as yours, and I do likewise.

In parallel planning, I plan all of our actions and you do the same.
I want to suggest that shared agency sometimes requires only parallel, and not interconnected planning.
Some of you are probably already thinking that the very idea of parallel planning is incoherent, and I will face up to this objection.
But first I want you to suspend disbelief and consider how parallel planning could enable us to coordinate our actions and our plans ...
Suppose you and I are parents about to change our baby's nappy.
This involves preparing the baby and preparing the nappy.
You're holding the baby and I'm nearest the pile of clean nappies, so there's a single most salient way of dividing the task between us.
Preparing the baby is, of course, a complex action ...
Now there are relational constraints on how the baby and nappy should be prepared; how you clean constrains and is constrained by how I prepare the clean nappy (because we don't want to get pooh on it).
\textbf{How do we meet these relational constraints?} The fact that I have a plan for the whole thing and so do you, and the fact that these plans are identical or similar enough that the differences don't matter means that your plan for your actions is constrained by your plan for my actions, which is my plan for my actions. So thanks to our parallel planning---to the fact that we each plan the whole action---your plan for your actions is indirectly constrained by my plan for my actions; and conversely. So: in parallel planning, we meet these relational constraints not by thinking about each other's intentions but by planning each other's actions.
There's just one tiny problem. In supposing that we both make a plan for the whole action, I'm implying, of course, that we each make plans for actions that are not our own. And this seems incoherent, unless perhaps we (the agents performing the action) are irrational or ignorant. It seems incoherent because the elements of plans we make are intentions; so, apparently, in making a plan for your action I would end up intending your actions. But I can't intend your actions, I suppose. What can we do?
The process of planning some actions is a process of practical reasoning.
As Millgram says, ‘Practical reasoning is reasoning directed towards action: figuring out what to do, as contrasted with figuring out how the facts stand' \citet[p.\ 1]{millgram:2001_practical}. Planning is practical reasoning in this sense. It aims to determine means given ends, and to select means for various ends in such a way that all of the means can be implemented together.
Note that in practical reasoning one has a special perspective on the actions; one illustration of this is that you ignore particular agents' biases and quirks, even if these are highly reliable. These can inform prediction but not planning processes. This will be important later.

without joint action

agent-neutral practical reasoning concerning actions that another will eventually perform resulting in no practical attitude

Our concern is with practical reasoning for jint action but first I want to step back from joint action and think about practical reasoning without joint action.
Practical reasoning can occur concerning actions that another will eventually perform. This can be useful for making predictions. To illustrate, suppose that you're having some work done in your flat and have hired an electrician and a decorator. The decorator arrives first. Realising that she will need to work around the electrician, she needs to work out how the electrician will approach the job. Because the decorator is also a skilled electrician, she makes a plan for the re-wiring of your flat. This process of planning has the key features of practical reasoning: it is a matter of figuring out what to do, it involves determining means given ends, and it involves selecting means for various ends in such a way that all of the means can be implemented together.
Now you might say that what the decorator is doing is not really practical reasoning. But note that it is preparation for action. Suppose you find out that the electrician can't make it, so you ask the decorator to stand in as the electrician. She does not have to engage in further planning or practical reasoning; she is already poised to act. So the practical reasoning that she has already done for actions that were to be performed by someone other than her puts her in a position to act immediately.
Even if you want to say, for some reason, that reasoning concerning actions one believes others will perform cannot be practical, you should recognise that it has some important features of practical reasoning: it serves to put one in a position to act, it involves determining means given ends, it involves selecting means for various ends that can all be implemented together, and it involves adopting a perspective insofar as particular agents' biases and quirks are ignored. These are the features of practical reasoning that matter for my argument.
You might object that it is incoherent to suppose that the decorator is really making a plan for actions she believes will eventually be performed by the electrician, but note two things ...
First, her planning process is agent-neutral. That is, it does not involve specifying any particular agent or agents. So she is planning actions that, she believes, another will eventually perform; nothing more than this is involved in the idea that she is planning another's actions.
Second, planning another's actions need not result in a practical attitude. (A practical attitude is an intention or something like it; world-to-mind direction of fit.)
The decorator's planning results in her having certain beliefs about the way in which some goals could be achieved. Much the same process \emph{could have} resulted in her forming intentions, but her belief that the actions will eventually be performed by someone else prevents her from forming intentions in planning.
So that some of your practical reasoning concerns actions that others will eventually perform does not entail that you have practical attitudes towards their actions.
(Just here we have to be careful with terminology. I am claiming that the decorator makes a plan for actions she believes the electrician will eventually perform. I am not claiming that the decorator plans or intends to perform those actions. Note also that to say the decorator `has a plan' for goals she believes are not her responsibility may be misleading: she has beliefs about how these goals could be achieved (so has a plan in one sense) but she does not have intentions concerning these goals (so lacks a plan in another sense).)

in joint action

What happens in joint action? I want to distinguish two possibilities ...

practical reasoning concerning my actions plus predictions about yours

We can imagine a situation where I think what I should do, then predict what you will do given that I do what I plan, then update my plan, then update my predictions, then ...
When I do this, I am conceiving of your actions as merely opportunities to exploit and constraints to work around.
(This is consistent with meeting the conditions Bratman offers for shared intention.)

vs

agent-neutral practical reasoning concerning our actions resulting in (which attitude?) --- dilemma

Alternatively, I can engage in practical reasoning concerning what we should do.
This sounds incoherent, but recal that I can also engage in practical reasoning concerning another's action for the purpose of predicting it.
And note that there is reason to think that these two different ways of reasoning about what to do might, in some cases, yield different conclusions (hi-low: don't explain).
Of course, this requires that the plan I make is agent-neutral.
And it requires that pratical reasoning doesn't invariably result in my having a practical attitude to the whole plan. But, if that's right, what attitude could I have towards this agent-neutral plan?
If the agent-neutral plan is to result in me acting, then I need to have some sort of practical attitude towards it.
But since some of the actions it specifies are, I know, actions you will eventually perform, I can't simply intend to implement the plan.
So here I seem to face a dilemma.
[First horn: not intention]
It seems that I can't intend it in the ordinary sense because then I'd be intending its parts, and some of its parts are actions that, I know, you will eventually perform.
So, apparently, I'm blocked from intending the plan
[Second horn: not belief]
On the other hand, if I merely entertain a belief about the plan --- if, for instance, I merely believe that the plan identifies a way we could achieve some goal ---
then it's unclear how I could be acting on it at all.

open-ended intentionsTo solve this dilemma we need to appeal to some ways in which intentions can be open-ended.: whatIt's a familiar idea that intentions can be open-ended with respect to what is intended.For instance, you can intend to visit the Weinachtsmarkt without intending to do so on any particular day., and whoIt's also true that intentions can be open-ended with respect to who will act on them.Consider a couple planning some tasks at the start of the weekend: they need to buy bread, to clean the bath, ... At this point, their intention is that one or both of them will do each of these things, but there is no further specification concerning who will act. Now you might say that you can't intend something without settling who will act. But this seems wrong given that (i) the couple's attitudes are practical, and (ii) generate requirements concerning agglommeration. (Even before it's determined who will do what, I know that I'm not going to be able to spend the afternoon in the pub.)[*skip] You might also say that open-ended intentions generate pressure to filling in details. This is true, but the details are not always filled by further intentions. At some point intentions give out and we just act. The point of appealing to the table-moving example was that here there is no need for the intention to specify the agents.I want to suggest that appeal to the open-endedness of intentions will help with the dilemma I had.The problem was, what attitude could I have to another's actions?

The attitude I can have is this: with respect to the whole plan, I intend that we implement it.
And with respect to its components, I intend that you or I or we do it.
So my intentions don't specify who will do what.
But they don't need to, because this is already adqeuately specified by the fact that you're holding the baby and I'm nearest the clean nappies.
So in this case what determines who does what are the constraints, not the intentions.

shared intentional agency consists, at bottom, in interconnected planning agency of the participants.’

(Bratman 2011, p. 11)

parallel planning

You plan my actions as well as yours, and I do likewise.

This is why, despite appearances, I think the notion of parallel planning is coherent. Without irrationality or ignorance, it is possible for us each to plan all of our actions, yours and mine, and to act on these plans. In doing so we achieve coordination and manifest collective intentionality not by thinking about each other's plans but, more directly, by planning each other's actions.
Let me return for a moment to the Tarantino walkers and what I was calling the Simple Account of shared intention. According to the Simple Account, we have a shared intention that we walk just if we each intend that we, you and I, walk. Now, as you may recall, earlier I noted that this seemed not sufficient because we might have and act on such intentions while forcing each other to walk at gunpoint. It is this problem that Bratman uses to invoke interconnected planning. But actually we can see that the problem can also be overcome by invoking parallel planning.
Consider the view that for us to exercise shared agency in walking together it is sufficient that: \begin{enumerate} \item we each intend that we, you and I walk; \item we pursue these intentions by means of parallel planning (that is, we each plan all our actions and our plans match); and \item we each end up with open-ended intentions concerning the components of our plans. \end{enumerate}
This view rules out the Tarantino walkers (who each point a gun at the other) because pursuing an intention by means parallel planning means taking a practical attitude towards each other's actions. So, if my conditions are met, your pointing a gun at me would be almost like your pointing a gun at yourself in order to force yourself to do something you intend.
[*skip] One more thing about interconnected and parallel planning. Earlier I noted that interconnected planning is demanding in two respects: it's demanding with respect to which mechanisms can underpin shared agency, and with respect to which agents can exercise shared agency. By contrast, parallel planning is not demanding in these respects. In parallel planning, I don't make plans about your plans, I simply plan your actions. So it does not demand mechanisms which are capable of meta-planning, nor agents capable of understanding and thinking about others' plans. Recognising a role for parallel planning in shared agency may thus allow us to understand exercising shared agency might not presuppose deep insights into the nature of minds, and so tempt us to consider the conjecture that it is through acting together that we first come to understand other minds. But that is another talk ...
Now so far I've been arguing only that (i) the notion of parallel planning is coherent, (ii) that parallel planning enables us to coordinate our actions, and (iii) that appeal to parallel planning might be useful for explaining shared agency because it gives us a way of ruling out some counterexamples. But that doesn't, by itself, amount to showing that the notion of parallel planning can is any more useful than that of interconnected planning. For all I've said so far, it might be that both parallel and interconnected planning play a role in corrdinating actions, but neither can be used to give sufficient conditions for intentional joint action. Can I do better?

practical unity

Earlier I suggested that interconnected planning can't be what shared agency at bottom consists in because agents can have interconnected plans while thinking of each other's actions only as opportunities to exploit and constraints to work around, and so without conceiving of themselves as exercising shared agency.
Now I want to suggest that in parallel planning, we take a perspective that allows us to see our actions, yours and mine, as having a certain kind of practical unity.
To illustrate, first consider the case of a single individual.
Imagine someone committed to keeping two or more areas of her life apart, so that she tries to plan separately for each area of her life. When concerned with planning in one area, she treats ongoing and planned actions from other areas of her life almost as if they were the actions and intentions of another agent who is temporarily acting with her body. Actions from other areas of her life feature in her current planning only as constraints to work around or opportunities to exploit. Of course, many of her predictions about her own actions are based on plans she has made when thinking about other areas of her life. But she systematically avoids conceiving engaging in planning for actions that involve different areas of her life; she does not treat her actions as even potentially parts of a single, larger plan. So there is a kind of practical unity that she fails to conceive of the actions which make up her life as having. She never takes perspective one has on actions when engaged in planning for them with respect to all her actions. Instead, at each time she plans for just one area of her life and takes the perspective of an outsider on the other areas of her life.
As this illustrates, the ability to conceive of any our actions as potentially featuring in a single planning process matters partly because it allows us to see them as having a kind of practical unity.
This applies to how you conceive of others' actions, not just your own. Earlier, I argued that you can sometimes engage in practical reasoning for not only actions you yourself will eventually perform but also for actions that others will eventually perform. This shows that it is sometimes possible to take the sort of perspective on others’ actions that you would paradigmatically take on your own actions. %***thanks to Peter Fossey here: I am not suggesting, of course, that you thereby conceive of others’ actions exactly as if they were your own. But nor do you conceive of the others’ actions in quite the way you would conceive of the actions of just any other agent who happened to be passing by. Rather, you conceive of these actions as on a par with your own actions insofar as they all feature in a single planning process.
This is why it's plausible that we can give sufficient conditions for joint action by invoking parallel planning
With this notion of parallel planning in mind, I want to return to my counterexample to Bratman's view.
Recall Beatrice and Baldric who had interlocking plans.
Since (by stipulation) they each avoid conceiving of their actions as even potentially parts of a single planning process, at any point in time each is taking the perspective of an outsider on either her own or the other’s actions.
To see that there really is no shared agency, contrast these two with Caitlin and Ciaran who engage in parallel planning for J1 ...
Here are Caitlin and Ciaran. Each makes a plan for all the actions, the actions the other will eventually perform as well as the actions she herself will perform.
So there is a sense in which they see their actions as having a kind of practical unity, and for this reason their case involves a joint action.
I want to take two things from this.
First, the contrast between CnC and BnB strengthens the argument for my claim that BnB's case really doesn't involve shared agency.
Second, the fact that CnC do manifest shared agency also shows, by the way, that interconnected planning is not necessary for shared agency.

Joint Action

Parallel but Merely Individual Action

Caitlin & Ciaran’s making the cross hit the red square

Beatrice & Baldric’s making the cross hit the red square

Two sisters cycling together.

Two strangers cycling the same route side-by-side.

Members of a flash mob simultaneously open their newspapers noisily.

Onlookers simultaneously open their newspapers noisily.

conclusion

My question was, What distinguishes joint actions from parallel but individual actions?
Bratman attempts to answer this question by giving sufficient conditions for joint action. In the first part of my talk, I offered a counterexample to the sufficiency of these conditions. And, then, in the second part, I argued that we can give sufficient conditions for an event to be a joint action by invoking parallel planning.
None of this implies that there is no role for interconnected planning at all. One requirement for parallel planning is that two or more agents have matching plans. I've suggested that this matching can sometimes be achieved thanks to a combination of similarities in the agents' planning abilities and environmental constraints. But there will, of course, be many situations in which these factors are insufficient to yield non-accidentally matching plans. Perhaps, then, interconnected planning matters in part because it enables agents to non-accidentally make matching plans in some situations. On this view, then, interconnected planning is not essential for shared agency but it does dramatically extend the range of cases in which we can exercise shared agency.
Sometimes interlocking intentions and knowledge states are not enough. Sometimes no amount of forming intentions about others’ intentions and acquiring knowledge of such intentions is sufficient, all by itself, for shared intention. In some or all cases, joint action requires changing perspective to conceive of your own and others’ actions as the upshot of a single planning process.
This matters for understanding why many of us see joint action as important in our lives \citep[compare][p.\ 327]{Bratman:1992mi}. In exercising ordinary, individual agency, we sometimes take a perspective that allows us to see our own temporally scattered actions as having a kind of practical unity. Relatedly, exercising shared agency sometimes involves taking a perspective that allows us to see our actions, yours and mine, as having a somewhat similar unity. It is not just that our individual plans mesh. It is that we plan almost as if you and I were one. In exercising shared agency we two are practically one.

\section{Motor Representation in Joint Action}

\section{Motor Representation in Joint Action}

motor representation

There has been much work recently on the role of motor representations in joint action.
I want to finish by suggesting that the view on joint action that I've been offering suggests the possibility that understanding joint action may require reflection not only on shared intention but also on motor representation.
Let me start by stepping back and consider an individual action.
An agent moves a mug from one place to another, passing in from her left hand to her right hand half way [*demonstrate].
It’s a familiar idea that motor representations of outcomes resemble intentions in that they can trigger processes which are like planning in some respects.
These processes are like planning in that they involve starting with representations of relatively distal outcomes and gradually filling in details, resulting in a structure of motor representations that can be hierarchically arranged by the means-end relation \citep{bekkering:2000_imitation,grafton:2007_evidence}.
Processes triggered by motor representations of outcomes are also planning-like in that they involve selecting means for actions to be performed now in ways that anticipate future actions \citep{jeannerod_motor_2006,zhang:2007_planning,rosenbaum:2012_cognition}.
Now in this action of moving a mug, there is a need, even for the single agent, to coordinate the exchange between her two hands.
(If her action is fluid,
she may proactively adjust her left hand in advance of the mug’s being lifted by her right hand \citep[compare][]{diedrichsen:2003_anticipatory,hugon:1982_anticipatory, lum:1992_feedforward}.)
How could such tight coordination be achieved?
Part of the answer involves the fact that motor representations and processes concerning the actions involving each hand are not entirely independent of each other.
Rather there is a plan-like structure of motor representation for the whole action and motor representations concerning actions involving each hand are components of this larger plan-like structure.
It is in part because they are components of a larger plan-like structure that the movements of one hand constrain and are constrained by the movements of the other hand.
But how is any of this relevant to the case of joint action?
Outcomes are represented motorically not only in performing an action but sometimes also in observing another agent perform that action.
in observing others act
motor planning* for others’ actions occurs
This can happen not only when observing a single agent acting alone but even when observing several agents performing a joint action \citep{manera:2013_time}.
Manera et al (2013)
--- even when several others act jointly ---
These motor representations trigger planning-like processes in the observer much like those that would occur if the observer were actually acting,
and thereby sometimes enable you to anticipate the targets of others’ actions, so that it is almost as if you were covertly anticipating their actions by planning how someone in their situation would proceed \citep{ambrosini:2011_grasping,Costantini:2012fk}.
and enables us to anticipate their actions.
Costantini et al (2013)
Consider one agent who is engaged in joint action with another:
they are moving a mug from one place to another, passing it between their hands half-way through.
Motor representations concerning outcomes to which the other’s actions are directed,
and the associated planning-like processes,
can occur in these agents \citep{kourtis:2010_favoritism,kourtis:2012_predictive,meyer:2011_joint}.

Now these agents engaged in joint action are performing complementary actions.
In general, performing one action while observing (or even imagining) a complementary action produces interference,
and at least some of this interference is probably caused by motor representations related to the observed action \citep{kilner:2003_interference,ramsey:2010_incongruent_}.
Given this, we might guess that
when motor representations concerning the other’s actions occur in joint actions involving complementary actions,
these representations would interfere with the agents’ performance and so impair coordination.
In joint action
Kourtis et al (2010; 2012)
it can also occur
However, it turns out that the opposite is true, at least in some cases.

Motor representations concerning another’s actions can facilitate interpersonal coordination in joint action.
(This is true for a variety of joint actions including passing an object, ballistic actions and playing a piano duet. (\citealp[p.\ 9]{kourtis:2012_predictive}; \citealp{loehr:2011_temporal}; \citealp{novembre:2013_motor}; \citealp{vesper:2012_jumping}; \citealp{vesper:2013_our}).

But how might motor representations concerning another’s actions facilitate performance in joint action?
Vesper et al (2012);
and informs planning* for our own actions.
Novembre et al (2013)
At this point it may be natural to suppose that in joint action
there are two (or more) separate sets of motor representations and processes:
one for your own actions, another for the other’s (or others’)
Like this [image of two separate plans].
This is indeed how things are in some competitive actions.
In competitive actions there are sometimes planning-like motor processes concerning an opponent’s action as well as concerning your own \citep{sartori:2011_simulation}.
In competitive action
we sometimes plan* the others’ action and then our own.
Sartori et al (2011)
But there is evidence that in some joint actions, a single outcome to which the agents’ actions are collectively directed is represented motorically by each agent \citep{loehr:2013_monitoring,Menoret:2013fk,tsai:2011_groop_effect}.
But in joint action, sometimes
Tsai et al (2011);
there is one goal that we each represent
Loehr et al (2013); Ménoret (submitted); Vesper et al (in prep.)
There is also evidence that when agents are engaged in joint action,
they sometimes take into account future actions to be performed by others when choosing how to act now, and do so in much the way they would if they were performing the whole action alone \citep{meyer:2013_higher-order}.
Taken together, this evidence suggests that when an agent is involved in joint action, there is sometimes a single plan-like structure of motor representations concerning both her own and the others’ actions.

\textbf{In joint action, it is sometimes almost as if we each engage in motor planning for all of our actions.}
and we each make a single plan* for both of our actions
Meyer et al 2013
So my thought is that something analogous to parallel planning can involve motor processes rather than practical reasoning. Let me say one more thing to complete the picture ...
Motor representations concern not only bodily configurations and movements but also more distal outcomes such as the grasping of a mug or the pressing of a switch \citep{butterfill:2012_intention,hamilton:2008_action,cattaneo:2009_representation}.
Some motor processes are planning-like in that they involve deriving means by which the outcomes could be brought about and in that they involve coordinating subplans \citep{jeannerod_motor_2006,zhang:2007_planning}.
Motor processes concerning actions others will perform occur in observing others act \citep{Gangitano:2001ft}---and even in observing several others act jointly \citep{manera:2013_time}---and enables us to anticipate their actions \citep{ambrosini:2011_grasping,aglioti_action_2008}.
In joint action, motor processes concerning actions another will perform can occur \citep{kourtis:2012_predictive, meyer:2011_joint},
and can inform planning for one's own actions \citep{vesper:2012_jumping,novembre:2013_motor,loehr:2011_temporal}.
In some joint actions, the agents have a single representation of the whole action (not only separate representations of each agent's part) \citep{tsai:2011_groop_effect,loehr:2013_monitoring,Menoret:2013fk},
and may each make a plan for both their actions \citep{meyer:2013_higher-order}.
Earlier we considered what is involved in performing an ordinary, individual action, where an agent moves a mug from one place to another passing it between her hands half-way.
Compare this individual action with the same action performed by two agents as a joint action.
One agent takes the mug and passes it to the other, who then places it.
The joint action is like the individual action in several respects.
First, the goal to which the joint action is directed is the same, namely to move the mug from here to there.
Second, there is a similar coordination problem---the agents’ two hands have to meet.
And, third, the evidence we have mentioned suggests that in joint action, motor representations and processes occur in each agent much like those that would occur if this agent were performing the whole action alone.

Suppose the agents' planning-like motor processes are similar enough that, in this context, they will reliably produce approximately the same plan-like structures of motor representations.
Then having a single planning-like motor process for the whole joint action in each agent means that
\begin{enumerate}
\item in each agent there is a plan-like structure of motor representations concerning each of the others’ actions,
\item each agent's plan-like structure concerning another's actions is approximately the same as any other agent's plan-like structure concerning those actions,
\item each agent's plan-like structure concerning her own actions is constrained by her plan-like structures concerning the other’s actions.
\end{enumerate}
So each agent’s plan-like structure of motor representations concerning her own actions is indirectly constrained by the other agents' plan-like structures concerning their own actions
by virtue of being directly constrained by her plan-like structures concerning their actions.
In this way it is possible to use ordinary planning-like motor processes to achieve coordination in joint action.
What enables the two or more agents' plan-like structures of motor representations to mesh is not that they represent each other's plans but that they processes motorically each other's actions and their own as parts of a single action.

So how does the joint action differ from the corresponding individual action?
There are at least two differences.
First, we now have two plan-like structures of motor representations because in each agent there is a planning-like motor process concerning the whole action.
These two structures of motor representations have to be identical or similar enough that the differences don’t matter for the coordination of the agents’ actions---let us abbreviate this by saying that they have to \emph{match}.
The need for matching planning-like structures is not specific to joint action;
it is also required where one agent observing another is able to predict her actions thanks to planning-like motor processes concerning the other’s actions (we mentioned evidence that this occurs above).
A second difference between the joint action and the individual action is this.
In joint action there are planning-like motor processes in each agent concerning some actions which she will not eventually perform.
There must therefore be something that prevents part but not all of the planning-like motor process leading all the way to action.
Exactly how this selective prevention works is an open question.
We expect bodily and environmental constraints are often relevant.
There may also be differences in how others’ actions are processed motorically \citep[compare][]{novembre:2012_distinguishing}.
\footnote{\citep[p.\ 2901]{novembre:2012_distinguishing}: 'in the context of a joint action—the motor control system is particularly sensitive to the identity of the agent (self or other) of a represented action and that (social) contextual information is one means for achieving this distinction'}
And inhibition could be involved too \citep[compare][]{sebanz:2006_twin_peaks}.

conclusion

My proposal, then, is this. In both practical reasoning and motorically, sometimes agents are able to achieve coordination for joint action not by representing each others’ plans but by treating each other's actions and their own as if they were parts of a single action.
So perhaps joint action is not always only a matter of intention, knowledge or commitment: perhaps sometimes joint action constitutively involves motor representation.