4. Common Knowledge and Experience in Social Space

We act in an environment in which much is in public view, and you might very well think that perceptual facts that cannot be known in common cannot be known at all. For any account that aims to explain the public character of our perceptual surroundings, the notion of common knowledge will be of crucial importance. I have been suggesting that the most basic perceptual common knowledge is of a spatial kind: in order to demonstratively communicate about and act on what is jointly perceived, we must know where things are in social space. 

Despite its ubiquity and importance, explaining the possibility of perceptual common knowledge presents a challenge. It can seem inescapable to think about it in terms of what each individual perceiver must know about what each perceiver knows about a jointly perceived scene (Schiffer 1972). Once you go down that path, you end up with an infinite regress of levels of individuals’ knowledge that apparently all have to be computed if anything is to be known in common at all. This regress is threatening not only because our cognitive faculties are finite; the analysis also fails to capture quite spectacularly that the relevant perceptual facts are “out in the open”: that joint perceivers can know them directly in experience (Campbell 2005). The most promising attempts to avoid the regress point towards an externalist account, on which you try to think through the problem of common knowledge not in terms of relations between perceivers’ cognitive states but in terms of the character of the perceptual and epistemic constellation in and about which it is available (Campbell 2011, Wilby 2010). 

I try to give substance to this view by adopting Williamson’s (2000) recommendation that we think of knowledge as a factive mental state. On this view, knowing is more basic than believing truly: it ties the knowing mind in the most direct possible way to the facts. You can only be in the mental state of knowing if the proposition that forms the content of your mental state is true, and your mental state is individuated by the environment in which the fact obtains. If you draw on Williamson’s externalist analysis to elucidate the problem of perceptual common knowledge, the resulting view is that its subjects are in type-identical mental states that are individuated by an environment ordered by a social spatial framework. You can write out the proposition each perceiver knows about the location of a jointly perceived object as follows: 

            (PL)     “*That is the object of the speaker’s referential intention,” 

where “*That” designates the object at the location that is determined through social triangulation in demonstrative communication. There is no regress here because each perceiver’s mental state of knowing a jointly perceived object’s location is individuated by an environment that guarantees that the proposition known by each perceiver is commonly known. Once perceivers know (PL), they can perceptually extend their knowledge about the jointly perceived object: for instance, they can come to know in common, as you and I did in my previous post, that there are two slugs rather than one in the salad. 

Williamson thinks that knowledge is not what he calls “luminous”: once you relativize the individuation of the mental state of knowing to the environment in which it is entertained, you allow that you may not always know that you know. By contrast, it is a stricture on common knowledge that its subjects know that they have it; common knowledge must be luminous, or it doesn’t exist (Sperber and Wilson 1995). And it is luminous because perceivers always know in common where the object of their joint perception is. On Williamson’s view, we have no “cognitive home” in which all facts lie open to view. By contrast, joint perceivers do enjoy such an abode, albeit of a rather modest kind: in social space, it is always known in common, and thus luminously, where things are, and on that basis joint perceivers can come to know more. 

 This externalist analysis is naturally aligned with a version of epistemological disjunctivism (McDowell 2008) about the kind of perceptual experience that produces perceptual common knowledge. Only and all joint experiences justify, absolutely, the relevant claims about their objects’ perceptual properties to their subjects. The explanatory power of epistemological disjunctivism comes out in full force when you consider knowledge acquisition that proceeds on the basis of joint experience: perceivers can always demonstratively justify their perceptual claims about the jointly perceived object and its relevant properties to each other; only joint experiences can play this discursive role. The common-factor theorist (Wright 2008), who opposes epistemological disjunctivism, holds that perceptual experiences are individuated by mental contents: you can then be in a type-identical experiential state regardless of whether its object (if any) is as presented, and the prima facie justificatory power of a particular experience is not dependent on its relation to its object. But this view does not have any grip as far as joint experiences are concerned: such experiences’ epistemic function of making possible the discursive justification of perceptual claims between joint perceivers, which is vital for the extension of their perceptual common knowledge, requires that the perceivers actually stand in the triadic constellation that is presented to them in experience. I cannot demonstratively justify a perceptual claim about an object to you if I mistakenly believe we are jointly attending to the thing while you are really looking elsewhere, or if I hallucinate the whole episode, including you.

Generally, if you think that communication and the kind of demonstrative justification by means of which joint perceivers can increase their stock of what they know in common about their environment have a serious role to play in our perceptual reasoning, you have excellent grounds for subscribing to a cluster of ideas that thinks of demonstrative reference in direct, common knowledge in externalist and joint experience in relational or disjunctivist terms. The core move is to shift the philosophical focus from the mind’s cognitive operations to the environment in and about which joint experience and common knowledge are possible. Because this environment is deeply social, such a move will begin by taking as given the sociocognitive relations in which joint perceivers stand to their surroundings. Only so is it possible to explain, I think, how we can point out to and learn from each other how things stand in the world we share. 

Campbell, J. 2005. “Joint Attention and Common Knowledge.” In Joint Attention: Communication and Other Minds, edited by N. Eilan, C. Hoerl, T. McCormack and J. Roessler, 287 – 297. Oxford: Oxford University Press.

Campbell, J. 2011. “An Object-Dependent Perspective on Joint Attention.” In Joint Attention: New Developments in Psychology, Philosophy of Mind, and Social Neuroscience, edited by A. Seemann, 415 – 430. Cambridge, MA: MIT Press.

McDowell, J. 2008. “The Disjunctive Conception of Experience as Material for a Transcendental Argument.” In Disjunctivism: Perception, Action, Knowledge, edited by A. Haddock and F. Macpherson, 376 – 389. Oxford Oxford University Press.

Schiffer, S. 1972. Meaning. Oxford: Oxford University Press.

Sperber, D., and D. Wilson. 1995. Relevance: Communication & Cognition. Oxford: Blackwell.

Wilby, M. 2010. “The Simplicity of Mutual Knowledge.”  Philosophical Explorations13 (2):83 – 100.

Williamson, T. 2000. Knowledge and Its Limits. Oxford Oxford University Press.

Wright, C. 2008. “Comment on John McDowell’s ‘The Disjunctive Conception of Experience as Material for a Transcendental Argument’.” In Disjunctivism: Perception, Action, Knowledge, edited by A. Haddock and F. Macpherson, 390 – 404. Oxford: Oxford University Press.



    I think you’re trying to deal with a very important problem – how can we share knowledge of the world? – but I think you’re making heavy weather of it. It’s difficult to impossible to understand you some of the time, because one has to wade through so much jargon and there is little plain talk. For example, you say you prefer to deal with common knowledge “in terms of the character of the perceptual and epistemic constellation in and about which it is available” Huh? No idea what that means.

    Let me have a crack at what you seem to be trying to say, for the sake of clarity —

    the reason we are able to share common knowledge is that we know that even though we may have radically incompatible if not opposite opinions and points of view , we are nevertheless “talking about the same things” , i.e. the same object(s) that exist in the world We know that even though we may be perceiving very differently – you through a rosy halo-ed filter, me through a horrific fragmented filter – we are perceiving nevertheless the same objects. And our points of view can be – and are, hundreds of thousands of times – tested directly, physically upon those objects. You say he took his knife with him, I say it’s still in the bedroom – we can both go into the bedroom or to see him.

    Common knowledge then is possible because people know, and endlessly test that, the *objects* of their knowledge exist in the real world, irrespective of their different to incompatible ideas about them. We know that “everyone has a different point of view” (especially in philosophy and science). But some points of consensus can nevertheless sometimes be achieved.

    Perhaps most interesting is why there is any problem at all here. It should be obvious that knowledge is of real world objects . But philosophy and our culture have to a considerable extent disconnected knowledge from the objects of knowledge, the words (and other symbolic forms of knowledge) from the world. I think part of the solution may lie in thinking of all forms of knowledge and thought as *reflections* (however vague and indirect) of the world. Then we start to become aware that there can be no reflections without reflected objects. You can’t divorce the reflection in the mirror from the reflected object. Words and other symbols are in fact wedded to, and basically inseparable from, the world. And it is those reflected objects out there that we all share – and are aware we share – knowledge of.

    P.S. We only need to know shared objects are out there somewhere – not at specific locations – the objects may be moving about.

    • Axel Seemann

      Hi, thanks for your thoughts. You are right, the basic problem can be stated very simply: how can there be common knowledge of the objects of perceptual experience, given that we perceive them from different perspectives and thus in different ways? But as far as I can see, there is no simple solution to this apparently simple problem. Trying to solve it requires quite considerable work, some of which I’ve outlined in the posts of this week. Because I am compressing much in the limited space of these posts, they may not always make for easy reading. In any case, I don’t think I understand what you mean when you say that knowledge and thought are “reflections” of the world. No doubt some knowledge is acquired through reflection, and no doubt knowledge (if it is a mental state) has conceptual content that consists in the propositions that are known. But you need a story, I think, about what it means to “share” what you know perceptually. Just that story is what I am trying to develop, successfully or not. I assign a vital role here to knowledge of location, as you will have seen – that this location can change when the object of perception moves is not in dispute.


    I have difficulty with your difficulty re “reflections”(purely a question of communication). Let me try and explain again – it’s v. important. I’m saying, to begin with, that everyone knows practically that their continuous visual images of the world reflect it v. successfully, firstly because we are embedded agents continually reaching out for and touching and navigating what we see. We also know that others have more or less the same images because they visibly touch etc what we see at the same time. Right away, you have the foundation endlessly tested for shared knowledge. We know we can make mistakes, see things differently, but there’s no question that in general we’re all looking at the same objects and are capable at least sometimes of agreeing about some of the nature of those objects.

    However for 2000 years philosophers have had difficulties with whether things/objects really exist – and knowledge can be shared. This is to a great extent because their archetypal human has been a philosopher/academic sitting in a room reading texts physically separated from the objects of those texts. This underlies all the main philosophical parables from Plato’s cave to Mary’s B/W room & the Chinese translator room. This in turn is because we have been a textual civilization in wh. the main form of knowledge was texts – words (& other symbols) being God, and visual/sensory images being v. secondary. And it is not at all clear, and still unsolved, how words and their underlying concepts connect to the world. The fact that we are embedded agents and not just academics, continually going out of our rooms/studies and into the external world, and testing our visual/sensory images empirically as scientists do, has largely escaped philosophers’ notice. Not only do we test our visual images, but we also continually test our word knowledge. Unbeknownst to philosophers, language is half prescriptions and not just (supposedly logical) descriptions . (Note BTW how you tend to assume with other philosphers that cognition is first and foremost a matter of testing (logical/verbal/textual) propositions). We continually verbally brief each others’ actions – PICK UP THE CUP, GO TO THE SHOPS, BUY ME SOME.., CLEAN THIS ROOM … and by God, we demonstrably obey each others’ word prescriptions. Our words are demonstrably jointly connected to – and in some way jointly reflective of – the world. Philosophers stuck intellectually in their caves are not aware of this.

    Yesterday, moreover, textual civilization died & has de facto been replaced by multimedia civilization, wh dramatically changes everything incl. science and philosophy, although philosophy hasn’t yet understood and absorbed the inevitable change. Our unimedium book of the world, in wh. the word was God and images were v. secondary, has been de facto replaced by our multimedia screen of the world, wh. is fast becoming also a console of the world.

    Two major consequences. One is that words and symbols and their truth/falsehood are no longer the almost be-all & end-all of cognition. They remain central but now also become dependent on and complementary to other forms. And we become aware that the foundation of cognition is exactly what nature and our screen say it is – movies/ the movie of consciousness. ( The most important thing about movies is not so much their “truth” (as in textual civilization and philosophy) as their “realism (level & extent of). Of course most of what we see in our movies is real. It’s no longer a serious problem. Not only do we physically test its reality, but we are now formally wired in via our screen/console to the world, and we can formally via our computer make what we see, do things. Turn lights on and off. Move objects around electronically. We can see in a self-aware way that we individually and jointly see and manipulate the same objects/world successfully. And we are now going to ask vastly more sophisticated questions about the realism of our images of the world than philosophers’ purely verbal and almost infantile “is snow white?” In a multimedia civilization it all depends on which *images* and which batches of snow, and not just which words, you’re looking at. Philosophers lost in words, words, words – empty words, hollow words disconnected from other media – never think to consider that. Now they will start to – and, I believe, your problem here re shared knowledge will largely disappear.

    The second consequence of multimedia civilizationis that we can now see that all our different forms and media of knowledge are interdependent and complementary and not alternatives – and are all “reflections”/images of the world, to very different extents and levels and diensions of realism. The search engine changes everything. We no longer look in the textual encylopaedia/dictionary for knowledge – for words about words. We now start by asking ourselves what objects *in the world* we want to know about, and which is the medium or media to search that is most appropriate to those objects – words, video, photos, audio… Which is the medium or set of media that is most *reflective* of what we want to know and see? (Search engines also make clear that we are all wired in to the same world, and wired very successfully, though all kinds of errors can occur. There is no longer any problem of shared knowledge).

    It becomes clear that words, logic, mathematics, numbers, graphs, photos, videos etc all capture/*reflect* different dimensions of the world in different ways and to different extents. Man cannot live by words alone. It becomes clear that all our words while vital are massively limited in their powers of reflection of the world. Words can only give us vague ideas/maps about the world. If we want to know the ultimate, most reflective truth about objects, we will have to go, if possible, to movies of them – even though the movies in turn with their purely local vision also require the global vision/maps that the concepts of language provide. We can now start to understand why nature insists that the movie of consciousness is never switched off, much as science insists that all our words be based on *evidence*, aka movies. Movies are the most reflective/realistic medium. We can start to see this, even though philosophers have never professionally heard of movies, and automatically assume that all images are still images (as in texts).

    Hope all this makes my message clearer re reflections. But it’s a massive subject area embracing all our culture’s forms of representation/reflection of the world, and our personal forms of cognition, so I’m still wrestling/groping here.

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