Organs

It is easy enough to come up with criteria which could be set down in addition to qualitative differences in sensations. We have already noted that the organs associated with the head senses are easy to identify and, following Armstrong, it can be asked: “What is our concept of a sense-organ? One mark of a sense- organ is obvious: it is a portion of our body which when stimulated produces a characteristic range of perceptions”.38 Identifying and classifying sense organs is the basis for important insights into the counting problem.

As soon as the physiological function of the nerves and brain in perception is recognised, it may seem natural to base studies of perception on anatomical and physiological studies which identify and classify different types of sensory organs and receptor cells and the associated groups of nerves.

To begin with, this approach can also reinforce some of the traditional understanding. Eyes and ears are very clearly identifiable organs and the optic and auditory nerves are, crudely judging by size, important pathways to the brain. Pursuing this approach further shows neurophysiology to be a revelatory study.

The human nervous system is exquisitely intricate and much of the body is enervated in some way. Considering this without prejudice undermines defi- nitions of sensing as perception of things and eventsoutside the physical body.

Although it is true that some sense organs are oriented in ways that facilitate interaction with the bodily environment, there is no unproblematic way to de- fine ‘external’ in connection with sense perception since conscious experience typically depends on not only collecting data about physical objects beyond the boundary of the skin; it is founded on the experience of embodiment, i.e.

37_{Examples to be discussed include Keeley (2002) and Gibson (1968).} 38_{Armstrong (1968, p. 212).}

bodily self-awareness.39

Physiological investigations are not only useful for elucidating the detailed mechanisms underlying the functioning of organs such as eyes and ears, they also reveal the sensory aspects of bodily processes—most importantly the per- ceptual processes involved in moving and maintaining position and posture. Hence it is not surprising that new senses have been added to the traditional five by physiologists investigating nerve functionality.

A number of different taxonomic issues arise from the new discoveries and it is not too much to say that confusion and disagreement generally follow. The simplest and most direct approach might seem to be to classify the organs or receptor cells and use this as a basis for counting the senses. This, however, only shifts the taxonomic problem to a new domain, that of biological structure and functioning.

A couple of examples of the kinds of decisions that have to be made are appropriate. Firstly, it is not difficult to argue that we have not one but two distinct systems for vision—one adapted for daytime colour vision and the other for nocturnal vision.40 _{It is true that these share an organ, but they}

work differently and need not have evolved concurrently. Should we count one or two vision systems? Animals with multiple vision systems not sharing the same eyes are not unknown, so answering this question is not as easy as may first seem. Further, there are three kinds of cones, the photosensitive cells needed for colour vision. Are there multiple colour organs?41

In a similar vein, cutaneous sensations of hot and cold are actually obtained from two different kinds of receptor cells—one only for hot, the other for cold.42 Are these then two senses? And are they separate from the senses we use to detect the relative warmth of internal organs? The suggestion that, for the case of hot and cold, the qualitative likeness or continuity between the sensations can be used to decide only reintroduces a reliance on the subjective experience, and sooner or later this poses problems related to the partiality of our acquaintance with our own sensations.

39_{The role of the body and the importance of sensorimotor skills is emphasised by No¨e} (2004), but the importance of these factors is already made clear in the work of Gibson (1968). A classic statement of the importance of embodiment is by Merleau-Ponty (2002).

40_{On such considerations see Ings (2007), especially his chapter 9.}

41_{Again, answering is not trivial once it is noted that individuals who are totally red-green} colour blind can distinguish some colour values at dusk which are indistinguishable for them in daylight. For information in various vision systems and problems with them see Ings (2007).

The neurophysiological approach left to itself generates very long lists of senses served by highly specific organs. Vision uses four receptors, taste four or five,43 _{touch (cutaneous or superficial pressure) an apparently indetermi-}

nate number,44 _{smell a bewildering array of specialised cells numbering in the}

thousands, and there are many more left to count when the other senses are examined. If this process is followed to its logical conclusion, the number of senses that can be distinguished and counted seems to be limited only by the skill of the anatomist.

The problem here is evidently the insistence on using biological structure and functioning exclusively, and the general response is to group various re- ceptors into systems—often called sense modalities—based more or less on the list of traditional senses. To this list are usually added a few new members but there is no clear agreement on what these should be. Nor is there any easy way of resolving these issues.

While this procedure may certainly be adequate for the scientist or the doctor needing to treat certain impairments, it is not particularly helpful for the understanding of philosophical aspects of perception. It might be hoped that neurosensory impairments may, for example, allow us to isolate what

information or what aspects of sensory experience are furnished by certain parts of the sensory-nerve-brain system. The trouble here is that some sen- sory disturbances have effects so far-reaching—involving not just memory and time perception but also the sense of agency and self—that they implicate in perceptual functioning all sorts of things which might be hoped to be quite separate.45

The discussion thus far of subjective experience and bodily organs has evidently left out one important—some might say the most important—way of resolving taxonomic issues. The objects of perception have not been discussed explicitly. These are considered next.

43_{Taste may yet be revealed to be more complex, with discriminations of the taste of fats} reported recently.

44_{Gibson mentions nerve “terminals [. . . ] of many sorts, not easy to distinguish”—in Gib-} son (1968, p. 106)—while in an authoritative textbook of human physiology it is explained that “at least six entirely different types of tactile receptors are known, but many more similar to these also exist”; Guyton and Hall (2000, p. 540). The situation is not helped by the observation that some kinds of receptors yield sensations of pressure, warmth, pain, etc. Incidentally, free nerve endings are associated with pain.