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How, if at all, are discoveries about game theory and its limits important for philosophical theories of joint action? In this lecture we investigate how team reasoning entails the existence of aggregate subjects. We also learn how to construct an account of shared intention using team reasoning. The result contrasts with Bratman (2014)’s account of shared intention.

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This lecture depends on you having studied some sections from a previous lecture:

For the minimum course of study, consider only this section:

The lecture is about applications of team reasoning to theories of aggregate subjects and shared intentions.

Main Objective

We will investigate how to construct an account of shared intention using team reasoning following Gold & Sugden (2007) and Pacherie (2013).

The upshot is an account of shared intention incompatible with Bratman’s account (see Bratman on Shared Intentional Action). This incompatibility, and the difficulty of picking a winner, is an obstacle to combining philosophical and formal approaches. We are therefore motivated to consider whether either there is a good objection to either account—or, alternatively, a way of making them consistent with each other.

As background, we will also introduce the notions of plural subject and aggregate subject. These are useful for distinguishing and relating different response to The Problem of Joint Action. Whereas Bratman (2014)’s theory requires neither plural subjects nor aggregate subjects, theories based on team reasoning do require aggregate subjects.

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aggregate subject : A subject whose proper parts are themselves subjects. A paradigm example would be a Portuguese man o' war (Physalia physalis), which is an animal that can swim and eat and whose swimming and eating is not simply a matter of the swimming or eating of its constituent animals. Distinct from, but sometimes confused with, a plural subject.
game theory : This term is used for any version of the theory based on the ideas of Neumann et al. (1953) and presented in any of the standard textbooks including. Hargreaves-Heap & Varoufakis (2004); Osborne & Rubinstein (1994); Tadelis (2013); Rasmusen (2007).
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, 2009; Pacherie, 2010; Brownell, 2011; Sacheli, Arcangeli, & Paulesu, 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.
motor representation : The kind of representation characteristically involved in preparing, performing and monitoring sequences of small-scale actions such as grasping, transporting and placing an object. They represent actual, possible, imagined or observed actions and their effects.
plural subject : Some subjects who are collectively the subject of an intention or other attitude. If there is one token intention that is both my intention and your intention and no one else’s intention, then we are the plural subject of that intention. (The intention is therefore shared in the same sense that, if we were siblings, we would share a parent.) Distinct from, but sometimes confused with, an aggregate subject.
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.)
team reasoning : ‘somebody team reasons if she works out the best possible feasible combination of actions for all the members of her team, then does her part in it’ (Bacharach, 2006, p. 121).
The Problem of Joint Action : What distinguishes doing something jointly with another person from acting in parallel with them but merely side by side?


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