Población

Suppose that SI is a state of the hum an visual module that is normally caused by square-shaped objects and rarely by objects of any other shape. In addition, suppose that whenever a subject tokens SI

she subsequently has a visual experience as of a square, an experience

w ith the content square. W henever a subject takes such an

experience at face value she subsequently behaves in a way that would be appropriate were there a square-shaped object before her, and inappropriate if otherwise. What is the content of SI? The

obvious answer is square (or square-shaped object). Now consider S2,

another state of the visual module. S2 tends to covary with circular­

shaped objects and causes visual experiences with the content circle

(experiences that, when taken at face value, manifest themselves in behaviour appropriate to circular-shaped objects). W hat is the

content of S2? The obvious answer is circle (or circular-shaped

object).

In a distant world inhabited by creatures intrinsically very much like us, optical laws and other external conditions are such that it is circular-shaped objects that typically cause tokenings of SI in the locals, and square-shaped objects that typically cause tokenings of S2. What is the content of SI and S2 in the inhabitants of this world?

One (externalist) answer is that SI has the content circle (or circular-

shaped object) whereas S2 has the content square (or square-shaped object). That answer suggests the following counterexample to individualism. Edgar is a normal Earthly subject who, confronted with a square-shaped object, tokens SI in response. Edgar has a twin who is an inhabitant of the above described distant world who is

currently confronted with a circular shaped object (an object that has caused him to token SI). The thought is that Edgar's SI has the

content square, whereas the corresponding state in his twin has a

quite different content (namely circle). Thus, the content of the states

of the visual module is not locally supervenient.^^

Is this a convincing counterexample to the individualist's thesis? I will argue for a negative conclusion; Marr would (or should)

attribute the content square to SI in both us and our other worldly

counterparts, which would thus imply that the latter systematically misrepresent the shape of objects in their world. There are two basic argum ents for this conclusion, one of which appeals to visual experience, and the other of which appeals to behaviour.

Here is how the argument from visual experience goes. The visual module plays a prominent role in the etiology of visual experience, so that how the world is presented to me in experience (the content of my experiences) will depend upon how my visual module

represents the world to be. If I have a visual experience as of a square,

then that experience will be the product of my visual m odule’s representing its distal stimuli as being a square-shaped object. Ceteris paribus, had my visual module represented the distal stimuli as having some other shape, then I would not have had the visual experience that I in fact had. In general, then, there is a systematic and coherent relationship between the output of the visual module and the content of visual experience. Consequently, part of Marr's task is to explain how the visual module manages to detect and represent those features of the world that our visual experiences represent. Marr will fail in this task if the contents that he attributes to the states (particularly the output states) of the visual module aren't appropriately related to the contents of visual experience, for he will have left it a mystery how the visual module enables us to see the world as we see it.

The role that the visual module plays in the etiology of our visual experiences (or, alternatively, in enabling us to see the world as we see it) implies that if the contents of the respective visual experiences of twins are identical then so will be the contents of the states of their

This twin scenario is closely based upon one developed by McGinn (1989) in his discussion of the question of whether the content of visual experience is locally supervenient.

respective visual modules, for any divergence in the contents of the latter states would show up in visual experience. In connection with

our specific example, if Twin-Edgar's experience is as of a square,

then the state SI in him must have the content square despite the

fact that that state covaries with circularshaped objects. To deny this is to make it a mystery how Twin-Edgar came to see the world as he

saw it; if SI had the content circle then surely he would have had a

visual experience as of a. circle.

As yet, none of this causes the externalist any problems, for it hasn't been established that the visual experiences of twins have identical contents. However, the assertion that the content of visual experience is locally supervenient has considerable intuitive power. Visual experiences are states of consciousness; they have

phenomenological or qualitative properties. There is something that

it is like to have an experience as of a square. Now w hat seems undeniable is that the qualitative character of an individual's psychological states is locally supervenient; the psychological states of twins are qualitatively identical no matter how much their home environments diverge. In other words, what it is like to be you is determined by how things are within your skin. Therefore, the qualitative character of Edgar's visual experience when he sees the square shaped object will be the same of that of his twin when he is confronted by the circular-shaped object. Consequently, if the content of a visual experience supervenes upon its qualitative character, then, given the transitivity of the supervenience relationship, the externalist's case collapses. And it has to be noted that it is intuitively highly plausible to claim that if someone has a visual experience

with the same qualitative character as my experience as of a square,

then that experience must also be as of a square regardless of the

nature of the home environment of the subject in question. Quite generally, it would take some argument to unseat the intuition that the content of an individual's visual experiences supervenes upon the qualitative character of those experiences and thus upon her intrinsic physical properties. I am not going to get embroiled in the question of whether such a supervenience relationship holds^^ but

McGinn (1989) argues for the thesis that the content of visual experience supervenes upon qualitative character. Davies (1992) disagrees. For him, the supervenience relationship is of the within world-within species variety.

merely note that visual experience is a potential source of trouble for the claim that the square-circle case constitutes a counterexample to individualism. To recapitulate: if the content of visual experience is locally supervenient then Edgar's visual experience will have just the same content as that of his twin. And if their visual experiences have just the same content then so will their respective tokenings of SI, given the role that the visual module plays in enabling us to see the world.

My central argument for the conclusion that the square-circle case does not constitute a convincing counterexam ple to the individualist's thesis involves an appeal to behaviour. The basic idea is that the behaviour of Twin-Edgar towards circular-shaped objects (being just the same as that of his Earthly twin towards square shaped objects) suggests that he systematically misrepresents them as

squares. Hence, SI has the content square in both twins,

environmental differences notwithstanding. In arguing thus I follow in the footsteps of both McGinn (1989) and Segal (1991).

Visual systems are not merely onboard entertainment systems. They perform a very important function, namely that of providing the subject with information about the nature of her world, information that it is useful for her to have if she is going to satisfy her needs and desires and generally prosper in the world. In order to perform this function it is important that visual systems are reliable; their pronouncements as to the nature of the external world must generally be veridical. But it is equally important that the subject is able to act on the basis of how she sees the world. In general, how we see the world shows up in our behaviour. Moreover, it is typically the case that how we behave on the basis of our visual states is appropriate given the content of those states. Thus, if I see an object before me as having a square shape, I will behave towards it in a manner that would be appropriate or sensible were it in fact square­ shaped, but not if it were any other shape. In behaving thus, my

behaviour is coherently related to the contents of my visual states. i

Consequently we can, and often do, read off facts about the contents |

of the visual states of our fellows from facts about their behaviour. If j

a creature couldn't behave in a manner that was coherently related to I

the contents of its visual states then there would be little point in its having a visual system; its visual system would play no role in

enabling it to prosper in the world. And if a creature could, but generally didn't, behave in a way that was coherently related to the contents of its visual states, then it might as well not have a visual system.^^

Now consider Twin-Edgar once more. He will behave in response to circular-shaped objects just as Edgar does in response to squares. For example, in describing the shape of the object before him when he is confronted with a circular-shaped object, he will do such things as trace a square in the air with his hand, or draw a square on a piece of paper. And he will attempt to circumnavigate circular shaped objects by following a square-wise path (a form of behaviour that will either result in him colliding with the object in question or expending excessive energy by going further out of his way than he need have done). In general, behaviour that state SI is implicated in the etiology of will be appropriate to squares (but not to circles), and that behaviour will be coherently related to the content of SI if that state

has the content square (but not if it has the content circle).

The upshot of all this is that if Twin-Edgar's behaviour is coherently related to the content of his visual states, then SI will have the

content square, and he will be systematically misrepresenting the

shape of circular-shaped objects. Thus the externalist has to argue that this is a case where there is a breakdown in the relationship between visual content and behaviour. Here, behaviour pulls us in one direction, and etiology in the other, so that we must chose between the two (or, more accurately, determine which way Marr would jump).

In the case of Earthly humans, the subjects of Marr's reflections, there is no such conflict between etiology and behaviour; Marr did not have to make any decisions on cases anything like the one that

Of course we don't always behave in a manner that is coherently related to the content of our visual states. Sometimes intervening factors prevent us from behaving as we normally would, as when we lose control of our limbs through disease or injury. Sometimes we do not take our visual experiences at face value. And sometimes we behave in such a way as to conceal how we see the world from our fellows. Yet such phenomena are exceptions to a general rule. Indeed, in the latter two kinds of case, the subject's behaviour will be coherently related to the content of her visual states along with tliat of the beliefs and desires that are implicated in her behaving the way she does.

concerns us now. Thus we will find no precedents in his work. However, I think that there are good reasons for thinking that his approach is such that he would have sided with behaviour in this kind of case, and thus that he would have attributed the content

square to Twin-Edgar's state SI. Such an attribution has the advantage of cohering with commonsense, for I take it that most ordinary folk would accept McGinn's claim that "aftermath trumps etiology", and would thus view Twin-Edgar as being something of a Mr. Magoo.

In determining how the visual module works, Marr relies heavily on the behaviour of human subjects; for example, their behaviour in norm al perceptual situations, their behaviour in experimental settings, their behaviour in laboratory conditions where normal physical constraints have been violated, and so on. In so relying on behaviour, he operates on the assumption that behaviour can tell us an awful lot about the workings of the visual system, for example, w hat information processing problems it solves, w hat physical constraints it takes advantage of/w hat assumptions it employs, what features of the world it is sensitive to and represents, and so on. This implies that he assumes that behaviour can tell us an awful lot about the contents of the states of the visual module, for to represent that module as solving a particular information-processing problem, or as relying on a particular assumption, is to make a claim about the contents of the representations that it generates. Thus Marr assumes that there is a systematic and coherent relationship between the goings on in our visual module and the manner in which we behave, and, moreover, that there is such a relationship between the contents of our visual states and the behaviour that they subsequently cause. Indeed, it is difficult to see how Marr could proceed in any other way, for if he were to abandon his reliance on behaviour he would have no hope of constructing a viable theory of vision. It's not as if he can cut open our heads and see what is going on inside.

What Marr's reliance on behaviour (and his assumption that there exists a coherent relation between the contents of our visual states and our behaviour) suggests is the following. Since Twin-Edgar behaves in just the same way as his Earth bound twin, Marr would attribute just the same contents to his visual states. In particular.

Marr would take his behaviour vis-a-vis circular-shaped objects as evidence that he systematically misrepresented them as square­ shaped; to attribute any other content to state SI would be to ignore all the behavioural evidence.

But how does this cohere with the success-orientation of Marr's theory? It might be argued that in addition to his assumption about behaviour, Marr assumes that the visual module generally produces veridical representations. In the case of the Earth bound Edgar, these two assumptions can jointly hold true but not in the case of Twin- Edgar (and his fellows). So, in connection with Twin-Edgar, Marr has to abandon one of his assumptions, and who is to say that it would not be the one that there is a systematic and coherent relationship between the contents of his visual states and the behaviour that they issue in? Quite apart from the point that Marr can't afford to abandon his assumption about behaviour if he is to have any hope of uncovering the workings of Twin-Edgar's visual system, there is an answer to this challenge. Although Marr takes our visual module to be largely successful in correctly representing the world (and sets out to explain this success), it is no part of his position that visual modules must be, or are inevitably, successful in this respect. He is free to take it as an open empirical question just how successful a creature's visual system is, a question that can only be settled by observing the behaviour of the creature in question (Patterson, 1996). The behaviour of us Earth bound humans indicates that our visual states are largely veridical. But matters are somewhat different when it comes to Twin-Edgar and his fellows. With respect to circular­ shaped objects, they will systematically engage in a pattern of behaviour that is wholly inappropriate, resulting in their crashing into such objects, their dropping them, and general chaos. Heaven knows what calamities will ensue if Twin-Edgar attempts to go

crown green b o w l i n g . ^ 4 pg Marr all this would surely indicate that

The fact that Twin-Edgar's interactions with circular-shaped objects result in such disastrous consequences serves to block a certain line of response that the externalist might try to develop. The argument I have in mind runs thus. Because of the nature of his home environment, we shouldn't automatically assume that when Twin-Edgar traces a square with his hand (or draws a square) his behaviour has the same significance as it would have here on Earth. Perhaps Twin-Edgar's square tracing behaviour means circle, thus implying that SI could have the content circle without

Twin-Edgar and his fellows misrepresent the shape of circular- shaped objects, and that their visual modules are far less successful than ours. Moreover, he would take it as his task to explain where their visual modules go wrong, to explain why their visual modules get it wrong when they get it wrong. Therefore, I conclude that

Marr, in line with commonsense, would attribute the content square

to state SI in both Edgar and his twin.