Philosophical work on the senses has largely been concerned with taxonomic issues: What makes an event sensory? Under which sensory kind should that event be classified? Answering these questions requires criteria of individuation. These would enable us to determine whether an event is the same as (or different to) sensory events in general and whether it is the same as (or different to) sensory events of a specific kind. A criterion of the first sort would allow us to identify vestibular events as sensory events. This would justify the belief that vestibular processes are sensory processes. A criterion of the second sort would allow us to identify vestibular sensory events as being of a specific kind, i.e., distinctively vestibular sensory events. This would justify the belief that there is such a thing as a vestibular sense. Failing to provide a criterion of the first sort would force one to classify vestibular events as non-sensory. But even if one were able to determine that vestibular events are sensory, one would still require a criterion of the second sort to classify vestibular events as sensory events of a kind that is distinct from, e.g., visual or haptic events.
To expand on this last point: as Lenggenhager and Lopez so masterfully describe, central vestibular processes are inherently multisensory, and as a consequence there is scarcely a part of our sensory and cognitive life that vestibular processes leave untouched (see especially §2.2 of the target article). But then, if vestibular processes are implicated in so many sensory and cognitive processes, it may be most accurate to see vestibular processing as simply a common part of many processes, rather than as an independent sensory system. That is, one may begin to seriously consider the possibility that vestibular processing does not constitute a form of sensory processing of its own kind, but rather constitutes a form of processing common to various other processes that are themselves sensory. This is, in effect, an issue that arises from applying a criterion for individuating the senses that includes the physiology (and neurophysiology) of the entire system. One might not be forced to this conclusion if one used an alternative criterion (Macpherson 2011a, 2011b). But it seems that each of the criteria commonly discussed would generate their own problems. For instance, employing a more restrictive criterion that delimited sensory systems according to their peripheral sensory organs would face the issue of whether the sensory organs of the vestibular system ought to include or exclude the so-called “truncal” or “somatic” graviceptors (Mittelstaedt 1992, 1996; Vaitl et al. 2002). Similar issues would be faced when attempting to individuate the senses in terms of a distinctive proximal stimulus. Alternatively, one might individuate the senses by means of certain distinctive experiences: vision distinctively represents the brightness, hue, and saturation of colours; audition represents the volume, pitch, and tone of sounds. The natural candidates for the vestibular system would be experiences that represent verticality, rotation, and translation. But whilst it is certain that the vestibular system typically contributes to experiences of verticality, rotation, and translation, these are all experiences of a kind that can be had through visual sensation alone, or through a combination of visual, somatic, and proprioceptive sensation. Moreover, although vertiginous experiences are the hallmark of vestibular dysfunction, these are either experiences of rotation, which brings us back to the aforementioned issue, or they are more vaguely classified as pseudo-vertiginous experiences of dizziness that may have any number of non-vestibular aetiologies. Suffice to say that it may be surprisingly difficult to find appropriate criteria to justify the claim that there is such a thing as a distinctively vestibular sensory process.
The foregoing characterises what would be the typical philosophical approach to the vestibular system, qua sensory system. This taxonomic approach captures certain philosophical interests, but it is completely inadequate for the task of bringing out the significance of the scope of the vestibular system’s influence. An alternative, structural approach focuses on the role played by vestibular events in processes that exhibit a certain kind of structure, to determine the contribution of those events to that structure. Note that the structural and taxonomic approaches are independent, insofar as they have different epistemic goals. They aim to further our knowledge in different ways. The goal of the taxonomic approach is to determine whether, and if so why, there is a distinctive sensory system of a certain kind. The goal of the structural approach is to determine whether, and if so how, a certain kind of process contributes to a certain kind of structure. By assuming that one can identify processes as objects of study without first employing an exhaustive taxonomy, a structural approach can assume that there are such things as vestibular processes without any commitment to these processes being wholly distinct from others. And by tracking the varied yet systematic effects of vestibular processes, one can determine whether vestibular processes contribute to a certain kind of structure, irrespective of, whether or not the vestibular system is a distinctive sensory system. As vestibular processes are implicated in so many and various sensory and cognitive processes, the structural approach seems to be the most fruitful in terms of the amount we might learn. It also seems more fruitful in terms of the kind of knowledge we might gain. For we may learn nothing about how vestibular processes affect our experiential life by learning that vestibular processes may not be, in the final analysis, of a distinctive sensory kind. But we will certainly learn something about how vestibular processes affect our experiential life by learning that vestibular processes contribute to a certain experiential structure. Accordingly, I leave aside taxonomic issues in the rest of this commentary and focus on structural issues. Specifically, I focus on issues concerning the role of the vestibular system in providing a particular kind of structure to our experience of the body and the world, namely a perspectival structure.
To begin with, we need a preliminary analysis of experiential phenomena that exhibit perspectival structure. I will call these perspectival phenomena. In the next section, I offer a rudimentary analysis of perspectival structure, the aim of which is to show that perspectival phenomena are more differentiated than commonly recognised. In the following three sections, I propose three lines of empirical investigation. Each would attempt to selectively study perspectival phenomena through measurement and manipulation of vestibular processes. If the experiments proposed yielded interesting results, they would further our knowledge of how vestibular processes affect the perspectival structure of our experiential life. Accordingly, the overall aim is to demonstrate how an analysis of perspectival structure might fruitfully interface with empirical research and facilitate understanding of structural features of conscious experience that would otherwise be obscured.