«Taxonomising the senses Fiona Macpherson Published online: 30 October 2010 Ó Springer Science+Business Media B.V. 2010 Abstract I argue that we ...»
Philos Stud (2011) 153:123–142
Taxonomising the senses
Published online: 30 October 2010
Ó Springer Science+Business Media B.V. 2010
I argue that we should reject the sparse view that there are or could be
only a small number of rather distinct senses. When one appreciates this then one
can see that there is no need to choose between the standard criteria that have been
proposed as ways of individuating the senses—representation, phenomenal char- acter, proximal stimulus and sense organ—or any other criteria that one may deem important. Rather, one can use these criteria in conjunction to form a ﬁne-grained taxonomy of the senses. We can think of these criteria as deﬁning a multidimen- sional space within which we can locate each of the senses that we are familiar with and which also deﬁnes the space of possible senses there could be.
Keywords Senses Á Perception Á Experiences Á Phenomenal character Á Representation Á Proximal stimulus Á Sense organ 1 Introduction The senses, or sensory modalities, constitute the different ways we have of perceiving the world, such as seeing, hearing, touching, tasting, and smelling. But what makes the senses different? How many senses are there? How many could there be?
Of any creature we can ask:
(1) How many token senses does it have?
(2) What types are those senses?
F. Macpherson (&) Department of Philosophy, University of Glasgow, Glasgow, Scotland, UK e-mail: ﬁona.firstname.lastname@example.org 123 124 F. Macpherson Types are general kinds of thing, and tokens are instances of types. For example, in the word ‘‘sense’’ there are four types of letter but ﬁve letter tokens. This is because there are two tokens of the same type: ‘‘s’’.
Questions 1 and 2 are very different questions, and it is important to keep them separate. To illustrate what we would be asking if we asked question 1, imagine that we came across a creature very much like a human but for the fact that it had four eyes—one pair above another. Call the creature ‘‘Four-Eyes’’. Four-Eyes might have one sense of vision, as we do, with all four eyes contributing to it, as our two eyes contribute to ours. Or Four-Eyes might have two distinct senses of vision, with each set of eyes contributing to its two different visual senses. If the latter were the case, and if Four-Eyes had no other senses, then it would have only one type of sensory modality, but it would have two tokens of that type. (Of course, for all I have said, Four-Eyes might indeed have three or four tokens of the visual sensory modality type.1) In this paper I will not discuss in detail the question of how we should individuate token sensory modalities, although this is an interesting question to investigate.
Rather, I will focus on the question of how to taxonomise types of senses. What are the principles we should use for individuating the senses and how many actual or possible senses are there?
2 What types of senses are there?
Many people have thought that there are only ﬁve types of senses. For example, Aristotle, in De Anima, famously said that there are ﬁve and only ﬁve senses: sight, hearing, touch, taste, and smell.2 (He is talking here both about the number and kind of senses that humans have and the number and kind that animals have.) This view has echoed down the centuries, advocated by a number of scholars, most recently perhaps by Matthew Nudds who says that it is ‘‘obvious’’ that humans have ﬁve senses and that their having this number is a truth of folk psychology. Moreover, he thinks that it is not the case that ‘‘common-sense embodies the kind of protoscientiﬁc understanding of the senses which is liable to revision or replacement.’’3 Therefore, he holds that no amount of extra data from science could change our minds on the question of how many types of senses there are.
1 Grice (1962) considers a creature like Four-Eyes. However, he puts his imagined creature to different philosophical use than I do here.
2 See Book III, Chap. 1. It is reasonably clear that Aristotle was claiming that as a matter of fact there are only ﬁve senses, and, given the nature of the world as he took it to be (composed of elements, each of which had different properties), there could be only ﬁve senses. Thus, he was claiming that it is nomologically necessary that we have only ﬁve senses. He was not claiming that it is metaphysically necessary.
3 Nudds (2004, p. 35). On the same page, not only does he say that he has ‘‘not come across a good argument’’ for the idea that the folk notion of the senses is liable for revision, but he also says, ‘‘There have been authors who attempt to give a ‘scientiﬁc’ account of the senses, but they do nothing to show that they haven’t simply changed the subject. Whatever they are giving an account of, it’s not the senses as we commonly understand them’’ (footnote 11).
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The commitment to the existence of only a relatively small, speciﬁable number of types of senses—typically, but not necessarily, ﬁve—forms part of what I call the ‘‘sparse view’’ of the counting question.
The sparse view maintains the following:
• The number of possible sensory modalities is relatively limited.
• The sensory modalities are discrete.4 To say that the modalities are discrete is to say that all of the possible modalities are rather different and distinct from each other (not that the modalities cannot interact).
Should one believe the sparse view? I think not, for two reasons. First, there is evidence that many more than ﬁve sensory modalities actually exist. From these cases we can go on and extrapolate and thus come to believe that the number of possible sensory modalities is large.
Let us take the case of humans ﬁrst. Many senses beyond the Aristotelian ﬁve
have been attributed to humans. Some of the best candidates are:
• proprioception—comprising awareness of the position of the parts of the body, awareness of movement of the body and of how much force is required to move it5
• equilibrioception—the vestibular sense or sense of balance
• the vomeronasal system—detects pheromones using the Jacobson’s organ in the nose and is separate from the olfactory system.
These are good candidates, because, plausibly, although by no means indisputably, they have some features that people have speciﬁed as necessary or sufﬁcient for being a sense, such as having a dedicated sense organ, producing experiences with phenomenal character capable of being accurate and inaccurate, and being, at least partly, exteroceptive.6 Candidates for yet more human senses include distinctive pain, temperature and pressure senses instead of one amalgamated sense of touch. Scientists have found that there are distinctive receptors that detect temperature, pressure, and painful stimuli and that there are separate spots in the skin receptive to pressure, warmth, cold, and painful stimuli. This has been the main reason that has persuaded some people that there are several senses here. However, in addition to this, some people have thought that the experiences of pressure, temperature, and pain are fairly distinctive; that is, they have rather different phenomenal characters. For example, it is sometimes claimed that Plato thought that temperature perception was a sense separate from that of touch and also that he thought pain was distinctive, being a sensation or ‘‘passion of the soul.’’7 Moreover, he did this not because he knew of 4 It may be that no one has ever held the sparse view that I outline here, but parts of it have certainly been avowed, and the position serves as a useful stalking horse.
5 The term ‘‘kinesthesia’’ is sometimes used interchangeably with ‘‘proprioception’’ thus deﬁned.
However, sometimes ‘‘kinesthesia’’ is used exclusively as a term for our sense of awareness of the movement of the body, while ‘‘proprioception’’ is reserved for the sense of the body’s position.
6 Note that the vomeral nasal system does not produce experience with phenomenal character but it does possess the other features.
7 See Classen (1993, p. 2).
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the differences in physiology that we know of today, but did so at least in part on phenomenological grounds.
Whether these are good enough reasons to postulate many senses in this case is highly disputed. One might doubt that pain, temperature, and pressure are particularly phenomenologically distinct. There seems, for example, to be a phenomenological continuity between experiences of excesses of pressure and temperature and experiences of pain. We also have evidence that a physiological overlap exists between the sensors that detect pressure, temperature, and painful stimuli—in both normal and pathological conditions. For example, pain seems to be elicited by extreme pressure or temperature (both hot and cold), suggesting that the mechanisms underlying experiences of each are not separate. However, this result might be explained away because it is hard to stop intense pressure and temperature stimuli from stimulating adjacent pain receptors in the skin. Nonetheless, there is more persuasive evidence in favor of continuity. Experiences of cold or vibration can inhibit the feeling of pain, and tactile acuity is diminished by painful heat experiences. Indeed, there is evidence of ‘‘multireceptive’’ neurons that are responsive to two or even three of these allegedly separate modalities, which some commentators claim indicate that the allegedly separate modalities are integrated centrally in the brain.8 Critics of the aforementioned evidence may think that these interactive phenomena are merely similar to the McGurk effect—and thus think of them simply as intermodal interactions between different senses. So, unfortunately, such evidence does not clearly settle the matter. Moreover, appeal to phenomenology to settle these issues is not straightforward since phenomenal facts are notoriously subject to dispute. Thus, there seems to be a large open question about whether there is one sense of touch or multiple, distinctive tactile senses.
Other candidates that have been considered as being additional human senses include senses of hunger, thirst, wet and dry, the weight of objects, fullness of the bladder, suffocation and respiration, sexual appetite, and lactiferousness.9 Indeed, in their survey of the human senses, Rivelin and Gravelle have concluded that, ‘‘Five is obviously just not enough to account for the huge range of sensory possibilities of which the human species is capable; seventeen senses is probably a more accurate count.’’10 This number may be well beyond the number one should endorse, but their survey gives an indication of the number of candidates that one may have to consider.
Outside the human sphere, there are even more candidates in the animal kingdom for being senses in addition to the Aristotelian ﬁve. For example, pigeons and other birds seem sensitive to the magnetic ﬁeld of the Earth, which gives them a fantastic sense of direction.11 It has also been shown that trout can be trained to strike at 8 The evidence adduced here about touch is summarized in Craig (1996). Craig claims that temperature and pain processing are closely coupled structurally in the brain and that brain lesions rarely affect one without the other. The brain’s processing of pressure is structurally more distinct.
9 See Dallenbach (1939).
10 Rivelin and Gravelle (1984, p. 17) 11 See Hughes (1999).
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targets distinguished only by their position in a magnetic ﬁeld. Moreover, a distinctive sensory organ and sensory system have been identiﬁed in trout that detect magnetic ﬁelds. This evidence has led people to think that all of the conditions required for positing a magnetoreceptive sense in trout have been established.12 Many ﬁsh and sharks seem to have an electric sense. Sometimes this sense takes a passive form, meaning that the creatures can detect electric ﬁelds that exist independently of them in the environment. However, there is another active form of the sense where the creatures produce an electric ﬁeld and then sense changes to it.
Some ﬁsh use this active electric sense for navigation and to detect other living creatures.13 A further apparently distinctive animal sense is infrared (IR) detection. All pit vipers and some boid snakes have pits on their heads that contain cells that are sensitive to infrared light. The pits are organs distinct from the snakes’ eyes and nostrils and can be used to accurately detect prey when the eyes are covered.14 From this evidence, one can see that many good candidates exist for being a sense, distinct from the Aristotelian ﬁve.15 Even if we required further information about these cases before we conﬁdently asserted that they constituted senses, these examples suggest that there at least could be senses of many different kinds other than the Aristotelian ﬁve. The only way to resist this thought would be to claim, as we saw Nudds do earlier, that the folk psychological notion of the senses is such that, according to it, there are only the ﬁve Aristotelian senses and that this concept of the senses is such that it is not liable to revision or replacement by scientiﬁc discovery. Do we have good reason to believe that the folk conception of the senses is as Nudds claims? I think the answer is no.
One reason to think that the folk notion of a sense is not restricted to the Aristotelian ﬁve is that scientists are some of ‘‘the folk’’ and the number of senses that they recognize has frequently been altered. As we have already seen, the debate about how many senses there are is a present concern to scientists but the debate is not a modern phenomenon. The number of senses has been disputed throughout history by both philosophers and scientists. See Dallenbach (1939).