The set of organs enabling orientation and dynamical reference in humans is a complex assembly called the vestibular apparatus. The organs are closely associated with hearing and we have two sets, one located behind each ear and protected by thick layers of surrounding bone. They transduce the direction of gravity and also angular accelerations in the three orthogonal directions of space, and make this information available, through complex neuronal inter- connections and wiring, to the active perceiver.
Although there is a wealth of readily available information on our organs of balance derived from modern research, the most instructive approach to what we now need to know about vestibular functioning and how it relates to our other senses is historical.
Among the important names usually associated with the history of vestibu- lar research are Ernst Mach, whose philosophical fame rests largely on psy- chophysics and his radical relativism, Jean Pierre Flourens, the famous physi-
ologist who described the organs, and Jan Purkynˇe, who examined vertigo after rotation.8 These are all figures from the 19th century, but Nicholas Wade has
looked at the early history and there is an unsung hero there. It is William Charles Wells, who was a contemporary and compatriot of Reid, and he de- serves the lion’s share of the credit for the early experimental work on how rotation affects our vision.9
The point to notice about much of this early work connected to the vestibu- lar organs is that it is not actually about balancing. It is really about vision and how our vision depends on movements and accelerations. The subjects of the experiments on vertigo and nystagmus following rotation were either strapped to a chair and spun about, or simply turned till they got dizzy, as children like to do.
If our main interest is in normal healthy balance, then these performances are only a small part of the story. They tell us a lot about the interactions and conflicts between vestibular function and the vision system, but little of direct significance about balance, and especially agency. To understand the basics of balance, it is better to pay attention to the remarks of someone who does not appear in the historical accounts of the discovery of balance at all: Thomas Reid.
Reid’s explicit remarks on balance occur in a late essay on voluntary mo- tion. This essay appears in the collection calledThomas Reid on the Animate Creation.10 It is called “Of Muscular Motion in the human Body” and it was
read before the Glasgow Literary Society in 1795.11 Wells published his “Essay
upon Single Vision With Two Eyes”, with a description of the experiments on vertigo in an appendix, in 1792.12 Wells knew of Reid and made some comment on Reid’s ideas on vision from theInquiry.
This is what Reid said on balance:
This Power we have of perceiving the ballance of our Body is so like to our other external Senses, that it might very justly have been accounted a distinct Sense, if it had been so much reflected upon as to require a Name.
In each of the external Senses, there is an Impression made upon the Body or on some part of it, which by our constitution pro-
8Some of the history is covered in Hawkins and Schacht (2005) and Wade (2003b). 9See Wade (2000) and Wade (2003a).
10Wood (1995). 11Wood (1995, p. 28). 12Wade (2000, pp. 130–131).
duces a certain Sensation of the Mind, and that Sensation is by our Constitution accompanied with the Perception of something external.13
He also remarked on the importance and excellence of this sense: When we observe with what ease, and Grace those Motions are performed by those who are expert, and compare them with the Laws of Motion, we must be convinced that this Sense by which we perceive the least deviation of the Body from its Ballance, may by Use be brought to a degree of Accuracy which is hardly to be observed in any of our other Senses.14
Contained in these remarks are tremendous insights about the senses, espe- cially if we pay attention to the context, which is a discussion of voluntary movements. Here is a little more:
This sense of Ballance may be seen in a Child of two or three Months old. If sitting upon ones knee he begins to tumble, he im- mediately starts & endeavours to recover himself; But it is greatly improved by Use, in every Employment that requires its exercise; [...] This sense of our Ballance is produced not onely by the impres- sion made by the power of gravity but by any other Force which endangers the Ballance.15
Reid does make some remarks on vision in the same essay, but these are mainly to do with directing the eyes by means of the antagonist muscles—so he speaks of a balance in the nervous power of those muscles—rather than the cross modal effects studied by those investigating vertigo and imposed accelerations. He is primarily concerned with how active agents use the muscles and notes that:
There are however many voluntary Motions in which some previous Perception of the Understanding is necessary to direct us to the Motion which the occasion requires.16
13Wood (1995, p. 110). 14Wood (1995, p. 111). 15Wood (1995, p. 111). 16Wood (1995, p. 110).
Not only must we sense how muscles move, muscular exertion is the default state:
Although all voluntary Motion is performed by the Contraction of Muscles, we must not from that conclude that when no Motion is willed, the Muscles are inactive. The Exertion of Muscles is no less necessary to rest than to Motion. In every position of the Body excepting perhaps that of lying prone The reason of this is that there are so many Articulations in the Limbs, & in the Spine & Neck and these in a living Body have such Lubricity to facilitate their Motions that without the Exertion of Muscles, it would sink down to the ground like a Chain of many links. So we see a Man does if he is struck dead or deprived of all power of Muscular Motion in an instant.17
As already mentioned, a few years before Reid’s remarks Wells published an essay on vision. In an appendix called “On Visible Position, and Visible Motion” Wells speaks about balance. He starts by noting that:
In the estimates we make by sight of the situation of external ob- jects, we have always some secret reference to the position of our own bodies, with respect to the plane of the horizon; and from this cause, we often judge such to be at rest, whose relative places to us are continually changing; and others to be in motion, though they may constantly preserve, in regard to us, the same distance and direction.18
The concern here is with the judgement of visible motions. Wells talks explic- itly about bodily balance a little further on, saying:
What is there within us, to indicate these positions of the body? To me it appears evident, that since they are occasioned and preserved by combinations of the actions of various voluntary muscles, some feeling must attend every such combination, which suggests, from experience perhaps, the particular position produced by it. But in almost all the positions of the body, the chief part of our muscular efforts is directed toward sustaining it against the influence of its
17Wood (1995, p. 112), emphasis and punctuation original. 18Wells (1818, p. 69).
own gravity. Each position, therefore, in which this takes place, must be attended with a feeling, which serves to indicate its relation to the horizontal plane of the earth.19
Wells then immediately considers how it is that we see objects to be still, despite irregular motions of the body such as are experienced on a ship rolling and pitching. The point is that Wells is really interested in visible position and motion, and how perception of these relates to bodily motions.
Reid, in his essay, is not particularly interested in the perceptions of sight but in the control of bodily movements themselves. Three of Reid’s key points are:
1. Voluntary movements and efforts maintain balance and posture. The implication here is that in using this sense we are active, in that we participate as agents in generating the sensations felt. The perceiver and the actor are one and the same, and if we wish to entertain a passive model of perception such as placing the perceiver in a Cartesian theatre, then we have to allow them to get onto the stage because without their activity and participation, the show simply does not go on.
2. This sense has its own sensations. These sensations are bodily sensa- tions associated with muscles, and Reid did associate balance closely with muscular sensations. These sensations are also normally sublimi- nal, unless we are in imminent danger of falling, or are pushed, and need to act decisively to restore our balance. As Reid might have said, we normally pass over these sensations unnoticed and attend to our other perceptions as we pursue our goals. That does not mean that we are not doing anything in keeping balance. In fact we are always acting, and the sensations informing us of posture and movement are always present. 3. There is evidence of development. Watching infants and young chil-
dren, rather than normal adult functioning, is helpful. Infants spend much of their time trying to orient themselves and to control their move- ments. The triumph of this development is getting mobile, particularly in standing up and walking. Even later, we can become more skilled in performing various motions.
There is no need to play Reid off against Wells in a competition on these points. Wells made closely related remarks and here is an example:
Should the necessity of supporting the body against its gravity, by the actions of our voluntary muscles, be suspended in whole, or in part, our judgments of the situation of objects, with respect to the horizon, must become irregular and uncertain, notwithstanding any general habit we may have acquired from experience.20
The main reason why what Reid tells us is exceptional comes from a fourth point, and that is his remark that we should count the sense of balance as an additional sense and compare our achievements to the Laws of Motion. That is a very fine suggestion because it challenges our ideas about what a sense is.