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Minimal Models for Acting Together

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The Leading Theory of Acting Together (Bratman’s)

Philosophical theories about joint action are based on contrasts between acting together and acting in parallel but merely individually:

‘When we act together [...] we are not each simply acting in light of expectations of the actions of others while knowing that those actions of others depend on their expectations of our actions. [...] merely publicly walking alongside each other on a crowded sidewalk without colliding, while involving complex forms of mutual responsiveness, is not yet walking together in a shared intentional way. Can we articulate conditions that go beyond such strategic interaction and are sufficient for and illuminating of our acting together?’ (Bratman, 2014, pp. 1--2)

Bratman’s first step towards answering this question is to postulate shared intention:

‘A first step is to say that what distinguishes you and me from you and the Stranger is that you and I share an intention to walk together—we (you and I) intend to walk together—but you and the Stranger do not. In modest sociality, joint activity is explained by such a shared intention; whereas no such explanation is available for the combined activity of you and the Stranger. This does not, however, get us very far; for we do not yet know what a shared intention is, and how it connects up with joint action.’ (Bratman, 2009, p. 152)

The view that joint action involves shared intention is almost universal.

‘I take a collective action to involve a collective [shared] intention.’ (Gilbert, 2006, p. 5)

‘The sine qua non of collaborative action is a joint goal [shared intention] and a joint commitment’ (Tomasello, 2008, p. 181)

‘the key property of joint action lies in its internal component [...] in the participants’ having a “collective” or “shared” intention.’ (Alonso, 2009, pp. 444--5)

‘Shared intentionality is the foundation upon which joint action is built.’ (Carpenter, 2009, p. 381)

But what is shared intention?

Bratman’s theory of shared intention has two components, a functional characterisation and a substantial ‘construction of interconnected intentions and other related attitudes ... that would ... play the roles characteristic of shared intention’ (Bratman, 2014, p. 32).[1]

Bratman’s Functional Characterisation

Shared intention serves to (i) coordinate activities, (ii) coordinate planning, and (iii) structure bargaining.

Bratman also proposes a requirement: shared intentions should be inferentially and normatively integrated with ordinary, individual intentions.

Bratman’s Substantial Construction

Bratman claims that the following are collectively sufficient[2] conditions for you and I to have a shared intention that we J:

(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 (1a), (1b), and meshing subplans of (1a) and (1b); you intend that we J in accordance with and because of (1a), (1b), and meshing subplans of (1a) and (1b)

(3) (1) and (2) are common knowledge between us.’ (Bratman, 1993, p. View 4)

These conditions have been elaborated in later work (e.g. Bratman, 2014, p. 52 on the connection condition).

Why We Need a Minimal Model

Does Bratman’s account capture simple forms of joint action, such as those that appear relatively early in development? Carpenter argues that it does:

‘I ... adopt Bratman’s (1992) influential formulation of joint action or shared cooperative activity. Bratman argued that in order for an activity to be considered shared or joint each partner needs to intend to perform the joint action together ‘‘in accordance with and because of meshing subplans’’ (p. 338) and this needs to be common knowledge between the participants’ (Carpenter, 2009, p. 381).

But the hypothesis that one- and two-year-olds have shared intentions as characterised by Bratman generates a prediction: since a function of shared intention is to coordinate planning, children of this age should be capable, at least in some minimally demanding situations, of coordinating their plans with another’s.

Is the prediction supported? There is good evidence that even 3-year-olds’ abilities to coordinate plans are quite limited. For instance:

‘3- and 5-year-old children do not consider another person’s actions in their own action planning (while showing action planning when acting alone on the apparatus). Seven-year-old children and adults however, demonstrated evidence for joint action planning. ... While adult participants demonstrated the presence of joint action planning from the very first trials onward, this was not the case for the 7-year-old children who improved their performance across trials.’ (Paulus, 2016, p. 1059)


‘proactive planning for two individuals, even when they share a common goal, is more difficult than planning ahead solely for oneself’ (Gerson, Bekkering, & Hunnius, 2016, p. 128).

A Minimal Model of Joint Action

A goal is an outcome to which an action is directed.

An outcome is a collective goal of two or more actions involving multiple agents if it is an outcome to which those actions are collectively directed (Butterfill & Sinigaglia, 2022).

Minimally, a joint action is an event involving two or more agents where the agents’ actions have a collective goal.

In virtue of what do our actions have collective goals? If we answered this question by appeal to shared intention, there would be a threat of collapsing the minimal model into Bratman’s model. We therefore seek an alternative answer.

One possibility is that some collective goals can be represented motorically (della Gatta et al., 2017; Sacheli, Arcangeli, & Paulesu, 2018; Clarke et al., 2019). If so, it is possible that not only intentions but also motor representations can link our actions to collective goals (Sinigaglia & Butterfill, 2022).


collective goal : an outcome to which two or more agents’ actions are directed where this is not, or not only, a matter of each action being directed to that outcome (Butterfill & Sinigaglia, 2022).
connection condition : ‘the condition that specifies the nature of [the] explanatory relation’ between shared intention and joint action ... [T]he basic idea is that what is central to the connection condition is that each is responsive to the intentions and actions of the other in ways that track the intended end of the joint action--where all this is out in the open’ (Bratman, 2014, pp. 78--9).
goal : A goal of an action is an outcome to which it is directed.
joint action : Many of the things we do are, or could be, done with others. Mundane examples favoured by philosophers include painting a house together (Bratman, 1992), lifting a heavy sofa together (Velleman, 1997), preparing a hollandaise sauce together (Searle, 1990), going to Chicago together (Kutz, 2000), and walking together (Gilbert, 1990). These examples are supposed to be paradigm cases of a class of phenomena we shall call ‘joint actions’.
Researchers have used a variety of labels including ‘joint action’ (Brooks, 1981; Sebanz, Bekkering, & Knoblich, 2006; Knoblich, Butterfill, & Sebanz, 2011; Tollefsen, 2005; Pettit & Schweikard, 2006; Carpenter, 2009b; Pacherie, 2010; Brownell, 2011; Sacheli et al., 2018; Meyer, Wel, & Hunnius, 2013), ‘social action’ (Tuomela & Miller, 1985), ‘collective action’ (Searle, 1990; Gilbert, 2010), ‘joint activity’ (Baier, 1997), ‘acting together’ (Tuomela, 2000), ‘shared intentional activity’ (Bratman, 1997), ‘plural action’ (Schmid, 2008), ‘joint agency’ (Pacherie, 2013), ‘small scale shared agency’ (Bratman, 2014), ‘intentional joint action’ (Blomberg, 2016), ‘collective intentional behavior’ (Ludwig, 2016), and ‘collective activity’ (Longworth, 2019).
We leave open whether these are all labels for a single phenomenon or whether different researchers are targeting different things. As we use ‘joint action’, the term applies to everything any of these labels applies to.
meshing subplans : ‘The sub-plans of the participants mesh when it is possible that all of these sub-plans taken to­ gether be successfully executed.’ (Bratman, 2014, p. 53)
modest sociality : ‘small scale shared intentional agency in the absence of asymmetric authority relations’ (Bratman, 2009, p. 150).
outcome : An outcome of an action is a possible or actual state of affairs.
shared intention : An attitude that stands to joint action as ordinary, individual intention stands to ordinary, individual action. It is hard to find consensus on what shared intention is, but most agree that it is neither shared nor intention. (Variously called ‘collective’, ‘we-’ and ‘joint’ intention.)


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  1. Bratman’s theory has been refined and defended over more than two decades (Bratman, 1992; Bratman, 1993; Bratman, 1997; Bratman, 2009; Bratman, 2014). Here we consider just the core components. ↩︎

  2. In Bratman (1992), the following were offered as jointly sufficient _and individually necessary_ conditions; the retreat to sufficient conditions occurs in Bratman (1997, pp. 143--4) where he notes that ‘for all that I have said, shared intention might be multiply realizable.’ ↩︎