(formerly "The Senses and the Structure of Experience")

For the sake of handy labels, let me call these respectively the Content criterion, the Qualia criterion, the Stimulus criterion and the Sense Organ criterion. Before going further, it is worth mentioning a modern response to Grice’s quest for the criteria for distinguishing the senses, namely that “sense modality” may be a “cluster” concept, such that several criteria are relevant to the concept, but none necessary, and in different contexts, different criteria may be weighted differently.  Even if “sense modality”, or “vision” is a cluster concept in this sense, the question of which cluster remains relevant to contemporary discussions, since one of the main foci of debate on the distinction between the senses is, in effect, whether there are any circumstances in which qualitative character as distinct from content is relevant to the distinction—or, in other words, whether the cluster includes qualitative character.  Grice succeeds in showing that it is if he can show that there are some contexts in which qualitative character plays a deciding role.  And, of course, to show that qualitative character sometimes plays such a role is enough to show, among other things, that at least some sense experiences really do have it.

So-called sensory substitution systems create an interesting problem for the Stimulus criterion.  These are apparati which transform one sort of stimulus into another.  The most well known of these is Bach-y-Rita’s TVSS (Tactile Vision Substitution System), which detects light waves and produces pressure signals on a grid on the skin. Other mechanisms detect light and produce sounds or mild electric shocks.  In these cases it is an interesting question whether the stimulus is light waves or sound waves (/pressure/electricity).  It depends on whether we are inclined to include the mechanism itself as part of the perceiver.  There is no a strong reason either for this or against it, which means that there is no strong reason for or against saying that the wearer of these devices sees the obstacles in her environment as opposed to hearing them or feeling them.  It is, however, clear that visual sensory substitution systems are not, literally, a cure for blindness.  They are merely aids for the blind, and this is not merely a question of detail.  Although it is true that sensory substitution systems provide poorer information than are typically available through the eyes, I take it that it would be no less absurd to say that these systems are restoring partial sight to the blind, though clearly they are restoring certain abilities.

There are a number of ways we might, as per the intentional content criterion, try to draw the distinction between the senses using the contents of perception.  For a first attempt we might go through the list of properties we perceive and divide them up into five groups according to the modality that perceives them.  This won’t work, however, because a lot of properties—they even have a collective name, the common sensibles—are common to different senses.  These are mostly spatial properties—size, shape, location, and so on; but we might also include sweetness here, since arguably it is perceived by smell and by taste.

Another way is to distinguish each sense by a single property which is unique to that sense.  So, for example, what makes sight sight is that it is a perception of—among other things—colour; hearing is a perception of, among other things, sound.  The problem with this is that some senses perceive more than one property uniquely.  Touch, for example, includes the perception both of pressure and of temperature, uniquely in both cases.  The “unique property” criterion would make it mysterious why pressure and temperature aren’t considered unique senses.  And before you start to say that they ought to be, even sight, a single sense if there is such a thing, perceives more than one property uniquely—hue and brightness, to name two.  So this way isn’t going to work.

A third way to try to use the content criterion is in terms of a range of properties.  This is actually Fred Dretske’s proposal.  According to this, we identify a sense modality with a unique range of properties perceived.  So, for example, we characterise sight by the fact that when we see we are aware of size, shape, location, movement, hue, brightness and saturation, and maybe a few more. The different lists for the different sense modalities will overlap, particularly in respect of the common sensibles, but there will be a unique list for each modality.

Of course when you are seeing and touching an object, and you close your eyes, even if you still perceptually aware of the objects’ shape by touch, clearly you no longer see its shape.  So there is still a detection link between the object’s colour and its visible shape.  But there doesn’t seem to be any way of characterising visible shape without presupposing the idea of a sense modality—at least if the only resources at one’s disposal are the properties perceived.  Whatever it is to be visibly square, presumably it’s to be square in the exactly the same way that tactile squares are square.  That is to say, there is only one relevant way in which the object itself can have the property squareness.  There is no such property as visible squareness as distinct from tactile squareness.

This is in fact Grice’s proposal, though he saw a significant problem with it.  If perception of some property is an instance of seeing just in case it has a particular feel—which is different to the property perceived—then it ought to make sense to say that the perception of exactly those properties we hear could have the feel that sight actually has, and we would in that case see sounds.  Grice had the strong intuition—one I think most share—that whatever is the relationship between content and character, it can’t be that easy to change the modality of a perceptual experience.

Grice did, however, argue that despite this difficulty, a difference in phenomenal character, irrespective of the content of a perception, could mean a difference in modality.  The basis of his argument is a thought experiment. In this thought experiment, we are asked to imagine meeting a Martian who looks very like the typical human, with the single exception that it possesses four eyes—one pair in the normal place, and another immediately above on the forehead.  The pairs of eyes are physiologically the same, and mediate the perception of the very same properties: colours, shapes, and so on.  However, it happens that the Martian uses different verbs to describe seeing through the bottom pair and seeing through the top pair—say, x-ing and y-ing.  Moreover, and crucially, when asked whether x-ing and y-ing feel any different, the Martian responds “Oh yes, there is all the difference in the world!” 

In this case, Grice claims, we should surely say that x-ing and y-ing are different senses, and therefore that the question of whether there is an introspectible difference between two perceptual states is indispensable to the question of whether they are in different modalities, and consequently that introspectible character is in general relevant to the distinction between the senses.

Finally, the Sense Organ criterion has been defended by a number of people, on a number of grounds.  Here I focus on a recent argument by Brian Keeley.  Though I will argue that Keeley’s argument fails, I go on to propose an alternative type of sense organ account.

Keeley (2002) arrives at the sense organ criterion via a process of elimination.  He writes that the problem of distinguishing the senses isn’t merely an abstract philosophical problem; it is also a biological one, discussed at length by biologists in relation to the star-nosed mole.  It is controversial whether this mole’s nose—or what looks like a nose—is part of a sense of smell or a novel sense, such as an electrical sense.  This problem looks very much like Grice’s original problem, namely of how to assess a claim that a creature possesses a novel sense, so it is very interesting to see how biologists actually try to solve it.

For our purposes the most significant thing they do is to dismiss psychology entirely.  The move isn’t defended by the biologists who are party to the debate, but Keeley defends it for them, in the following way. In order to use the contents of experiences to differentiate the senses, we need to distinguish direct from indirect content.  For example, although the perception of temperature is if anything a kind of tactile sense, when we see a glowing red hot plate we often say that it looks hot; alternatively we might say of a rose that it looks fragrant.  We need to be able to ignore these indirect contents if we are to be able to use the content criterion.  Grice also saw this problem for the content view and Keeley endorses Grice’s solution, which is to make the distinction in terms of qualitative character; so, to directly perceive redness is for redness to be part of the qualitative character of the experience.  In cases of indirect perception there is no associated qualitative character.  So, in order to distinguish the senses in virtue of the contents, we need to be able to refer to the phenomenal character of the experiences (Dretske 1999, at least, has since addressed this problem, successfully I take it, by introducing a distinction between property-awareness and fact-awareness; but I grant the point here for the sake of the argument).

The problem, then is this: in the case of the star-nosed mole, for familiar reasons it is impossible to say what the character of its experiences are; or, crucially, whether the mole even has experiences with phenomenal character.  But even if we supposed that the mole in fact has no phenomenal experiences, that doesn’t seem to dissolve the problem.  It still makes sense to ask whether the mole’s nose is part of a sense of smell or of electricity.  Therefore neither psychological criteria will work, leaving the sense organ criterion as the only one left standing.  This doesn’t mean that the problem is solved, since it remains to discover what distinguishes sense organs from one another.  Keeley gives an interesting and detailed account of what that distinction amounts to, but I won’t follow that up here.  Instead I want to focus on Keeley’s argument that psychology doesn’t really matter to the distinction.

It seems unarguable that our actual basis for judging of ourselves that we are using one modality rather than another is experiential.  Compare, for example, the difference between being touched on the tongue and being touched on the nose, with the difference between a sweet taste and a sweet smell.  In the former case the difference is simply a matter of location; but the latter difference is clearly more than that.  Or take the difference between feeling a vibration with one’s skin and hearing it.  Again, the fact that one is an instance of touch perception and the other is an instance of auditory perception is just obvious—the nature of the respective experiences makes it plain.

Some further evidence for this comes from cross-cultural research.  If the division into five senses is a constant across cultures, this would suggest that it is based more or less directly on experience. This is what we find.  According to Jütte (2005), the list of five senses that we are familiar with is the same list we find being taken for granted in records from ancient Greece, India and China. One would expect cross-cultural variation if the list we have of the five senses is compiled through even a small amount of theorizing (or even, for that matter, a small amount of arbitrariness).  Since this seems to be lacking, we can take this lack as support for the idea that the list is based more or less directly on experience.

Now it does happen to be the case, of course, that a difference in sense modality comes with some physiological difference or other at the periphery.  So there is perhaps some room for arguing that while, as it were, our “surface judgements” about sense modality are phenomenologically based, what those judgements actually end up picking out are the physiological differences—on analogy with the distinction between surface judgement about the presence of water and the chemical kind that those judgements actually pick out.  The big problem with this is that while the surface properties of water are also the surface properties of H2O, it is very implausible that the surface properties of a sense modality—by hypothesis its phenomenology—are the surface properties of sense organs.  Sense organs may cause experiences, but it would be a very odd view according to which sense organs themselves had experiential properties.

To this problem we can also add the implausible consequence of Keeley’s view that prosthetic devices cannot restore perception in a sense modality; at most they can add a new modality.  By all accounts, however, hearing implants really can enable a deaf person literally to hear.

So what about the star-nose mole problem?  Without access to its psychology, does this mean that the sense modality associated with its nose is inaccessible?  I think the answer is yes.  But there is an analogous problem that isn’t inaccessible.  Let me call this the Physiological Problem of the Senses.  This is more or less the question that occurs to us when we see the star nose on the mole, namely “What on Earth does that thing do?”  Is it a limb or a sense organ, and if the latter what information is it collecting and how?  These are all interesting questions to ask, and indeed they are the focus of the star-nose mole problem as it is discussed in biological circles.  But because none of them are straightforwardly linked to how, if at all, the mole becomes aware of whatever information is being detected, they are at most analogous to the traditional question of whether the nose indicates some novel sense modality.  Which is not, obviously, to say that the biologists’ questions aren’t fascinating question their own right; they are simply different questions.

What is the significance of this?  Note again that the main difference between Grice’s “martian” thought experiment and my variation is that in the former the Martian has two sets of eyes, and in the latter only one.  This is the key difference between the two scenarios, and it seems to make all the difference to our intuitions.  

Given that there are different sense organs involved, qualitative character really does make a difference to our intuitions about sense modality.  I think there is a reason that our intuitions are sensitive to this combination in particular; that there is a specific aspect of our experience which captures the combination—what I will call, for want of a better name, the feeling of using a sense organ.  

Writing of the “feeling” of using a sense organ will be provocative to some.  I might equally have used the term “awareness” rather then “feeling”.  Part of a perceptual experience is an awareness of the sense organ being used.  I use the term “feeling” to express some agreement with anti-representationalists who claim that a perceptual experience involves more than just awareness of the object of the perception.  Plainly, being aware when you are seeing that you are using your eyes is to be aware of more than what you are seeing.  The idea, to borrow a phrase of Gibson’s, is that perceiving is proprioceptive as well as exteroceptive, and that the senses themselves are distinguishable proprioceptively.

Indeed the idea that the phenomenology of an experience is directly informative of the sense organ involved has similarities with a position put forward by J.J. Gibson (1966).  Gibson is most well known for a thesis about the content of perceptual experience—that it is not directly of objects and properties but rather of possibilities for action, or “affordances” as he put it.  This is not the idea of Gibson’s that I have in mind.  One of his less radical proposals was that it is wrong to think of the psychology of perception in purely passive terms. Some our behaviours—some of the actions we perform—are distinctly perceptual. An obvious example is moving one’s eyes – that is to say looking around.  Gibson called these sorts of behaviours “exploratory behaviours”, and called attention to a history within psychology of noting this special class of perceptual actions, going back at least to Pavlov (1927), who termed them “investigatory responses”. He argued that each of the five senses involve a distinct set of exploratory behaviours, and that, moreover, for any perceptual experience, implicit in the experience itself is an awareness of the corresponding set of behaviours.

Now these different sets of exploratory behaviours have what we might call different foci.  In the case of vision, the focus of behaviour is the eyes.  The head is involved, of course, but clearly it is no accident that in the vernacular “to look” is “to use your eyes”.  We can roughly identify this focus of action with sense organs—not in the sense simply of the parts of the body that sense things, but rather as the parts of the body which we use to sense things.