WINNER OF THE PHILOSOPHICAL ESSAY PRIZE 2010: The simplicity of mutual knowledge

Abstract

Mutual perceptual knowledge is a prevalent feature of our everyday lives, yet appears to be exceptionally difficult to characterise in an acceptable way. This paper argues for a renewed understanding of Stephen Schiffer's iterative approach to mutual knowledge, according to which mutual knowledge requires an infinite number of overlapping, embedded mental states. It is argued that the charge of ‘psychological implausibility’ that normally accompanies discussion of this approach can be offset by identifying mutual knowledge, not with the infinite iterations themselves, but with the finite base which Schiffer proves is capable of generating those iterations. An understanding of this finite base as a primitive, relational property holding between two or more people, allows us to understand the iterations as an implicit and ‘harmless’ intrapersonal feature of what is an interpersonal phenomenon. The paper concludes by relating the account to joint attention in infant interaction.

Keywords:

• intersubjectivity
• social psychology
• cognitive science
• folk psychology
• developmental psychology

Acknowledgements

Thanks to Tom Stoneham, Mike Beaney, Matthew Ratcliffe, Chris Dowling, Anna Armstrong and the members of the Philosophical Explorations Essay Prize jury for helpful comments and suggestions.

Notes

See also Bach and Harnish (1979).

This is adapted, with a few changes, from Halpern and Moses (1990, 556; quoted in Syverson (2003, 46)).

I have followed Schiffer's convention of referring to a speaker and audience, although the account which follows can be thought to apply just as much to a mutually perceived event as to a mutually perceived utterance.

The term ‘generating property’ comes from Peacocke (2005). I am taking the term to refer to a finite, physically reducible property, true of an individual, that is sufficient for that individual to have knowledge of a particular fact. For instance, an agent facing a tree in broad daylight, has knowledge that there is a tree in front of him due to certain physically reducible properties that hold true of him: e.g. that he has a well-functioning perceptual system, is not cognitively damaged in any significant way, is standing at such-and-such a distance and angle from the tree, etc. I take it that it is a plausible working assumption that these properties can, theoretically, be cashed-out in purely physical terms.

Traditionally, the three major players have been the ‘iterative approach’ (Schiffer 1972); the ‘fixed point’ approach (Harman 1977); and the ‘shared situations’ approach (Barwise 1989).

At the beckoning of Microsoft word-check, I have used the third-person form of the verb, rather than the first person plural, so as to emphasise the notion of a genuinely plural subject. As J. David Velleman puts it ‘a truly “plural subject” ought to be a single subject that isn't singular – or, if you like, a plural subject that isn't a plurality of subjects’ (Velleman 1997, 30).

See, for instance, Gilbert (1989) and Eilan (2007). Campbell's (2005) approach is slightly different, because on his account the co-attenders are ‘present as constituent[s of each other's] experience’ (p. 292). What allows Campbell to make this claim, without lapsing back into the problems that arise when each participant is considered to be the object of the other's perception, is his rejection of a representational understanding of perception in general.

In slightly more (speculative) detail: I would suggest that the idea of a literally shared state of mutual knowledge, should be primarily understood as applying to mutual perceptual knowledge (including mutually perceived utterances), where the shared perceptual knowledge in question is understood as being conceptual, as opposed to non-conceptual (Gunter 2003). The further question of whether that perceptual knowledge should be understood as experiential (rather than purely propositional, a la Davidson) is dependent on the contentious issue of whether the experiential aspects of perception should themselves be understood as conceptual in any sense, a question which is beyond the scope of the present paper.

The claim here is that the appeal to a relational account suffices to give a straightforward solution to the problem of joint action, insofar as that problem arises in the General's Paradox. However, it should be noted that the General's Paradox does not appear to be primarily a problem regarding the conditions required for joint action as such, but rather a problem regarding the necessity of mutual knowledge for joint action (section 1). The proposal here, then, attempts to dissolve the problem of mutual knowledge that drives the General's Paradox, but is silent on what, if any, further matters of co-ordination might be required for an actual launch of a coordinated attack, beyond the mutual knowledge of a mutually understood signal.

See, for instance, Bratman (1993).

As we shall see in section 4, the idea that social-cognitive understanding has its basis in externalised, bodily/behavioural-based relations has a rich history in psychology, especially within developmental psychology. See especially the work in the Vygotskian tradition.

Again, for those who are getting impatient with this seemingly implausible claim, I urge you to postpone judgement until the discussion in section 4.

It is worth pointing out that the possibility that one of the Generals might get jittery about his knowledge does not thereby throw us back into the paradox, such that each must now be convinced that the other is not feeling jittery. The reason for this, is that the worries that such a General might entertain are negative, and do not encompass the state of mutual knowledge per se, but rather express an idiosyncratic need for a complete absence of doubt about that knowledge, and thus involve the prior assumption that there is a state of mutual knowledge towards which one might entertain doubts. To understand the problem of mutual knowledge as a problem regarding the absence of doubt about an item of knowledge (as Davies (1987) does), is to make the mistake of supposing that the problem of mutual knowledge is just a special case of the Cartesian search for philosophical certainty. However, as I argued in section 1, the problem of mutual knowledge does not arise from this traditional form of philosophical scepticism, but rather arises from the need to acknowledge a psychologically expedient state of mutual knowledge, with a role to play in psychological theory and in action. It is no objection to an account of mutual knowledge that the agents in question might have doubts about that mutual knowledge, or make mistakes with regards to it (perhaps one of the Generals just perversely cannot be bothered to attack, despite knowing the conditions obtain), just as it is no objection to the idea that one might have knowledge simpliciter about a particular fact, even though one might reflectively entertain doubts about it, or make a mistake with regards to it.

See especially Bruner (1990), Gallagher (2001), Hutto (2004), Carpendale and Lewis (2006), Ratcliffe (2007), Eilan (2007) and Striano and Reid (2009).

Barwise (1989) mentions game theory, law, and communication theory (p. 198), although he might also have mentioned economics and sociology.

For a more recent version of the worry, see Hutto (2008, 170–3).

Fodor (1987, 132), for his part, does not actually think this problem spells doom for the theory of cognitive development, because he embraces the idea that folk psychological understanding is innate, and thus not in need of development.