Proprioception and Naive Realism
Proprioception and Naïve Realism
Philosophical discussions of sense perception seldom address proprioception, but it is a question whether theories that hold for the other senses, particularly vision, hold for proprioception. The objects of proprioception are generally taken to be the muscles and joints, possibly also the skin; they do not include all bodily organs. We do not sense our bones in this way or our internal organs (with the exception of the heart and gut, which are muscular).  It might be said that we can perceive our bones (their size and position) indirectly via our perception of the muscles surrounding them, but the skeleton itself contains no proprioceptors. So the bones are not direct objects of internal perception; similarly for the brain, liver, and blood—or the fingernails, hair, and teeth. We know the properties of these things by means of the senses, but we don’t sense them by proprioception; they are not elements in our “proprioceptive field” (compare the visual field). Simplifying, let us restrict proprioception to the muscles: we sense our muscles this way but nothing else—not other people’s bodies or external material objects. The muscles are the proper objects of proprioception; and this is a sense like no other, a sense in its own right. It makes us acquainted with things in a unique way.
Various questions can be asked about this sense. Is there something distinctive it is like to deploy the proprioceptive sense? Are there proprioceptive qualia? Is it a type of feeling, analogous to touch? Do we sense events in our muscles as well as the muscles themselves (contractions, relaxations)? What properties of our muscles do we sense? Are the muscles perceived to have both primary and secondary qualities? What kinds of perceptual illusions is proprioception subject to? Is it possible to be a skeptic about proprioception? What is the relationship between proprioception and introspection? Do we sense the mind as we sense the body? How is our body image related to our self-image? Does proprioception provide the most basic kind of knowledge of our body? What would happen if we lost all proprioception? Is our conception of our body mainly determined by proprioceptive sensation? To what degree do we model the external world on the body as proprioceptively perceived?
These are all good questions, but I intend to focus on a different question: Is naïve realism true of proprioception? I hold that the other senses do not satisfy naïve realism in the following sense: their direct objects are not external distal material things but proximate arrays of energy or materiality.  We do not, strictly speaking, see, hear, smell, taste or touch external objects like tables and chairs, or stars and bananas. We see impinging light, hear ambient sounds, smell the chemicals that emanate from objects, taste the chemicals that stimulate our taste buds, and touch the force fields emitted by things (none of these objects is a sense-datum in the traditional sense, not being mental in nature). I will not defend this view here; my concern is to suggest that we dodirectly sense our muscles. Thus there is a fundamental asymmetry between proprioception and the other senses: naïve realism is true of this sense but not the other senses. If I look at my biceps muscle, I directly see the light packet reflected off it and I indirectly see the muscle itself; but when I sense my biceps muscle by proprioception I sense the muscle directly not some sort of energy emanating from it. There is no perceptual intermediary that is the immediate object of my perception. I am acquainted with the muscle itself not merely with the effects it has on my senses. Proprioception is logically unlike hearing (the paradigm sense in my view): we hear the sound an object produces not the object itself (save indirectly), but we don’t feel the “sounds” produced by the muscles—we feel the muscles themselves. When I apprehend my body I really do apprehend my body. There is nothing mediated or remote here, no dependence of one kind of perception on another; I don’t indirectly perceive my muscles in virtue of perceiving something else. In fact, there is nothing else that I perceive when I perceive my muscles—no analogue of light or sound or chemical impingements. It is the muscle itself that falls within my proprioceptive field not some sort of emissary or intermediary of it.
It is hard to argue for this position except by pointing to the absence of anything that could play the role of intermediary, and also perhaps its phenomenological verisimilitude. What could we proprioceptively perceive butthe muscles of the body? The case is rather like our “perception” of our own mind: we “sense” our thoughts and sensations directly not via some sort of intermediary. Mental states don’t reflect light or emit sounds or send out mental chemicals that enter a mental nose or mouth. Similarly our muscles don’t have these kinds of effects on our internal sense: their properties are immediately given in proprioception. We perceive our muscles somewhat as we perceive light or sound—that is, directly and immediately. But our muscles are solid objects like external objects, so these are the only such objects that we sense directly. I sense your body indirectly via my external senses, but I sense my own body directly (as well as indirectly through my other senses). Proprioception is unique in giving us access to physical objects directly. Thus there is nothing about such objects intrinsically that precludes them from being directly perceived; it is just that objects can reach our minds in different ways, some direct, some indirect. Mostly we sense the physical world indirectly, but in the case of proprioception we are granted a direct route.
This is not to say that perceptual illusion is impossible in proprioception, or that skepticism doesn’t apply; and indeed both are possible. We have the phantom limb phenomenon as well as the standard brain in a vat scenario. So we don’t know with certainty that we have muscles. The point is rather that the structure of our perceptual relation to the body in proprioception is unlike our other forms of perceptual relation to things: if we perceive our body (pace skepticism), we do so without perceptual mediation—without sensing something else first. It is not that we perceive our muscles by perceiving our skin or something of the sort (as we do when seeing our muscles). In this sense, then, we can be naïve realists about proprioception: we really do directly perceive the physical things called muscles. Of course, we have proprioceptive experiences that are distinct from muscles, but these are not perceptual intermediaries, since we don’t perceive them (we have them). The experiences make it as if we are sensing our muscles directly, and indeed that is true as a point of logical analysis: we literally experience our muscles.
It is an interesting fact that this asymmetry exists: instead of naïve realism being true or false of perception in general, we have the result that it is mainly false but not universally so. Different philosophical theories of perception apply in different cases. Interesting, too, that our own body should be the object that engages our perceptual apparatus most directly, so that our body becomes the thing most intimately experienced (not counting the mind itself—though the mind isn’t experienced). “Know thyself!” takes on a new meaning. And is it possible that we tend to model external perception on proprioception, regarding it as more intimate than it really is? In fact, there is a sharp logical discontinuity between perception of the body and perception of other physical objects, animate and inanimate. Other bodies resemble our bones in contrast to our muscles. 
 We can sense pain in internal organs that contain no muscle, and this might be regarded as a form of perception; but that is not the same as proprioception, which has to do with the perception of position, size, and movement. Thus sensory physiologists distinguish proprioceptors from nociceptors.
 See my “Seeing the Light” in Philosophical Provocations (MIT Press, 2017).
 Don’t our bones seem more alien to us than our muscles, as if they aren’t quite ours? Ditto for our internal organs (most of them). We are fonder of our muscles because we know them better. The skeleton is not an object of affection.
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