We have five senses and five types of sensory experience. This is doubly contingent: we might have had fewer or more senses, and we might not have had a different phenomenological type of experience corresponding to each sense. The second claim is less obvious than the first, but evident on reflection. First, note that the relationship between stimulus-type and experience-type is contingent: the physical nature of the stimulus doesn’t entail the phenomenological nature of the perceptual response. Thus you can’t infer what visual experience is like from the physical nature of light or what auditory experience is like from the physical nature of sound waves (similarly for touch, smell, and taste). Nor can you infer the physics of the stimulus from the nature of the experience. There is a lawful correlation between stimulus and response, but there is no identity or metaphysically rigid relation between them. One could exist without the other. This lack of necessity underlies some familiar thought experiments: we can imagine rerouting the inputs from the ears into the visual cortex, so producing visual experiences from auditory stimuli, and vice versa. Or there could be beings initially set up to convert sound waves into visual experience and light into auditory experience. The stimulus contains information about the environment and the brain interprets this by using alternatives modes of phenomenological response. Isn’t this what the human senses already do to some extent? The same (distal) stimulus can be seen or touched or even heard, and smell and taste respond to the same molecular stimuli. There is also the phenomenon of synesthesia, in which the same stimulus produces a response in two sense modalities. How the brain codes sensory inputs is not dictated by the physical stimulus, distal or proximal; in principle, we could invert the relations that actually obtain. There are possible worlds in which light produces olfactory sensations and people taste visually.
But there is a thought experiment of this class that I have never seen mentioned: the idea that all the senses of a given creature might be served by the same phenomenological type. For instance, our five senses might be centrally manifested in only visual experience—we only see things when stimuli impinge on the ears, skin, nose, and mouth. We reduce the phenomenological range to a single sensory type that is common to all the senses. For humans this set-up would require some major (futuristic) surgery, so let’s assume we are dealing with a Martian that is born this way. Symphonies are “heard” as complex patterns of shifting light, objects “feel” as they are seen, food “tastes” like a visual mosaic. For any type of input, there is just one mode of experiential response: instead of experience pluralism we have experience monism. Whether we should describe this situation as possessing a single sense that responds to a variety of stimuli or several senses that are mediated by a single type of experience is not critical to decide; what matters is that there is a leveling of phenomenology combined with the usual types of sensory impingements. The same variegated physical world is represented by a uniform type of phenomenal world. This seems like a logical possibility, not ruled out by the concepts or by some deep metaphysical necessity. Granted, we don’t find any actual instances of it on planet Earth, but there might be other planets that are home to beings like this.
This thought experiment raises interesting questions. First, is there some biological reason that we don’t find actual instances of creatures like this? On the face of it such a set-up is more parsimonious than the actual situation, and doesn’t nature prefer parsimony? The genes would only need to engineer a single type of central sensory nerve to handle input from all the senses—the visual type. This would serve as well, so why complicate the physiology? It may be retorted that representing all the senses in a single phenomenological type would be confusing for the organism, since it wouldn’t know whether it was tasting or seeing; but this could presumably be accommodated by assigning different visual types to the two sorts of stimulus. Isn’t this what we already do within the visual sense—as when we have distinct sensations for shape and color? Couldn’t the all-encompassing visual sense contain a reference to the part of the body being stimulated, so that it was clear what sense was being activated? Why should this be any more confusing than simultaneously receiving inputs from senses with different phenomenological character, since a central unit has to separate and integrate the inputs so received in this case too? The purely visual organism could be constructed so as to keep track of the origin of its visual experiences, in part by assigning different visual types to each type of input. Brighter colors might be assigned to one sense compared to another, or different colors entirely. Visual experience is already very various and dependent on varying aspects of the light stimulus, so there seems no problem of principle preventing a purely visual subject from existing (perhaps one that is perceptually simpler than us). More strongly, this might be a better way to increase sensory bandwidth: smell and taste might become more discriminating when mediated visually. To the objection that visual tasting wouldn’t have the motivational force of ordinary tasting, we could stipulate that gustatory visual sensations be genetically linked to the pleasure centers of the brain, so that certain visual arrays elicit pleasure in the hungry eater. Don’t some tastes become pleasurable to us that were once repugnant or bland? Why not have menus listing the particularly tasty color combinations on offer tonight? You bite into an oyster and your visual sensorium lights up with an accompanying rush of pleasure.
So parsimony recommends experience monism, but so do other aspects of the organism. Don’t we find a conspicuous absence of florid pluralism in the anatomy and physiology of the body? The bones are much the same in point of composition throughout the body, despite differences of function and structure—we don’t find different types of bone composition according to where the bone is located. What would be the point of that? It would just make ontogenesis more difficult. And the underlying physiology of the nervous system is likewise homogeneous: the nerves associated with the different senses are of basically the same type (a nucleus, axons, dendrites, and the same suite of chemical neurotransmitters); we don’t find radically different histological characteristics from sense to sense. Moreover, the distal stimulus is likewise uniform: the same physical world is present to each sense—consisting of atoms, forces, etc. But the sensory systems inject a marked heterogeneity into nature: they are more richly various from a phenomenological point of view than the external world or the physiology of the brain. They provide the pageantry and pizzazz. So we have a puzzle: why so much variety when parsimony and the general laws of nature recommend uniformity? Why make seeing so very different from hearing—or smelling so very different from touching? It seems like an act of generosity from nature to the experiencing organism—making life a little less boring and monotonous. But natural selection and the genes are not known for their generosity; they like things as simple as possible (such complexity as we find is forced on organisms by the rigors of survival). Our thoughts don’t exhibit as much phenomenological variety, no matter what their subject matter may be, so why do our senses insist on the gaudy plurality of our sensory experience? It seems surplus to requirements, a gratuitous gift, an unnecessary extravagance. What would you say if we had fifteen senses each equipped with its own distinctive phenomenology when far fewer would do just as well? That would seem like biological largesse above and beyond the cause of gene propagation; why not strip it down a bit? The natural thought is that the variety we experience must possess some hidden biological utility, but it is not clear what this utility is, given the informational powers of visual experience (or the other senses in their most advanced forms). The cell serves every biological purpose in the body, but it is fundamentally the same from organ to organ. To be sure, cells vary somewhat from heart to kidney, skin to brain, but no more than visual experiences differ among themselves. What we don’t find is organisms (or organs) made of completely different chemicals, or partly cell-based and partly continuous, or bones that are sometimes made of calcium and sometimes made of metal. We find variations on a theme: but sensory experience varies the theme. Seeing is really nothing like tasting. To lack a sense is to lack something sui generis, to miss out on something unique. A purely visual organism might go blind but still be replete with visual sensation; a blind man, however, can get at best hints of what vision might be like. Each type of sensory experience is, we might say, a world unto itself.
One possible view is that the present sensory set-up is temporary and the result of a dispensable holdover from earlier evolutionary times. The senses evolved separately as solutions to survival challenges and the senses that now populate the planet build upon these early forays (much the same is true of basic anatomy). This is not a matter of ideal optimality but of contingent evolutionary history. Conceivably, the process could have started with greater uniformity and stayed that way, or it might eventually work out the kinks and favor sensory homogeneity. If we were building sentient robots, we might be faced with a design decision—one type of central component that delivers only visual phenomenology or several types that afford sensory variety. The decision could affect future production whether or not we make the optimal decision. If reasons of economy favor the single-component approach, we might end up producing purely visual robots (though capable of responding to the full variety of physical inputs). This might correspond to life on other planets, depending on the actual course of evolutionary history. On our planet the earlier “decisions” favored distinct types of sensory experience, and thereafter organisms were stuck with them. This arrangement might be highly sub-optimal despite its universality in terrestrial life forms. If we imagine an early life form equipped only with visual sensations responsive to light, wondering how to expand into other stimulus fields, it would be intelligible if this form plumped for retention of its existing phenomenological capabilities extended to other types of stimulus. It could either devise new modes of sensory response to sound waves and other types of stimulus or stick with what it has onboard already. The latter choice might be preferable, given the engineering demands created by branching out. So we must not simply assume that experience pluralism is the biological ideal; it might just be an adventitious artifact of how evolution on earth has actually progressed. Aliens might view our mixed phenomenology as distinctly old-world, pre-self-technological, and recommend switching to a more streamlined approach (they promise it will not be boring compared to the cumbersome system we now employ). Or there might be hearing-obsessed aliens (of bat-like aspect) who urge the merits of their sensory world and disparage the purely visual species. After all, whoever said that we humans are biologically perfect? Surely pain is not the best possible way to cope with injury in every possible world, so why should sensory diversity be the best possible way to handle information in every possible world? Among the life forms of the universe it might be quite parochial. Certainly some life forms on earth manage quite well without the full panoply of the five human senses—bacteria, worms, and much marine life.
I will mention another possibility, if only for completeness. This is that our sensory phenomenology might be less various than we suppose. Obviously, introspection plays a determining role here—we experience ourselves as experientially plural. We seem to ourselves to contain phenomenological multitudes. But perhaps this appearance is misleading; perhaps we are more uniform than we think—as the external world is more uniform than we naively suppose given the way we experience it. From a more abstract or objective point of view we may be more uniform than we appear. We already accept that there are commonalities in perceptual experience—intentionality, spatial embedding, functionality—and it may be that there is a way of describing experience that will render it more unified than our current ways. A more objective phenomenology might be a more uniform phenomenology; there may be structural universals across sense modalities. Synesthesia suggests as much. Just as science can reveal hidden universals, so a scientific phenomenology might reveal experiential universals beyond our current grasp. Then the variety of sense experience would be revealed as superficial. Chomsky sometimes suggests that there is really only one human language when you get right down to it, despite superficial appearances; well, is it ruled out that there might be just one type of human sense experience? Call this Universal Phenomenology (UP for short): just like Universal Grammar, Universal Phenomenology might unite all human experience and distinguish it from other possible types of sensory awareness (reptilian, Martian). If that were so, phenomenology might be as uniform as physiology at a deeper level. I don’t think we could ever conclude that really there is just the sense of vision, with every other sense a minor variation on it; but we might conclude that the deep structure of all sensory experience is common to every type—no more various than the cell types that correlate with experience in the brain. At any rate, this is a possibility to keep in mind, especially since otherwise we seem confronted by a genuine biological puzzle (the puzzle of excessive phenomenological variety, as we might call it).
Our language is hooked up to our senses, so that we can comment on what we see and hear (etc.), but we don’t have a separate language for each sense equipped with its own sound system, syntax, and semantics. That would be pointless and biologically redundant, as well as confusing and energy-consuming. So why do we have separate phenomenological systems hooked up to our senses instead of a single system? Why isn’t our sensory system more like our language system? The language system is a singular and separate module with its own distinctive internal structure; it is not divided into five different modules each with its own grammar and lexicon. Evidently, this kind of architecture could in principle characterize our sensory system—say, a single visual module hooked up to our several sense organs. Yet that is not what we find, but instead a diverse and divided set of systems that must all be integrated somehow. It seems unduly complicated and unwieldy, like speaking five languages when one would suffice. Why the difference? Why not speak a single phenomenological language?
 This thought experiment emerged during a conversation with Tom Nagel on October 10, 2019.
 Here we might be reminded of Nagel’s discussion of “objective phenomenology” in “What is it like to be a Bat?” The more a phenomenological description prescinds from the specifics of a given type of experience the more universal it is apt to be. Thus we might aspire to cross-modality phenomenology.