One day a few months ago, I felt a tiny prick between my shoulder blades. I couldn’t tell what it was; it could have been an irritation of the skin, or something poking at me. It took no particular cleverness to find out, and what I did next is exactly what you would have done, dear reader. I squirmed and wiggled my shoulders. At this point, it became obvious that there was something sharp rubbing against my skin, for I felt it move as I wiggled my shoulders. Moreover, since the movement of the sharp point seemed to coordinate with the movement of my jacket across my back, it was clear that there was something sharp caught in the cloth. I took the jacket off, but could see nothing. But grasping the cloth with my fingers and feeling around with my thumbs, I finally detected what was wrong. A stiff thread had come loose from the padding. Looking closely, I was finally able to see it.
In the nineteen eighties, the philosophy of perception was transformed by the need to accommodate the perceptual constancies—that things look more or less the same colour in different lighting conditions; the same shape from different angles; the same size from different distances. We are able to estimate perceptual qualities that (on the face of it) do not show themselves directly in the information available to a perceptual system. Things sound as if they are far away, or moving, or in a place that can be visually identified—how can this be?
Now, many philosophers of perception (including myself) argue that these phenomena show that perceptual experience directly reveals a world outside the perceiver—it puts us in touch with non-perspectival truths. How we perceive things is to a great degree independent of how we are situated. You can dispute how this comes to be. The great German physicist, Hermann von Helmholtz, argued that constancy is learned; his compatriot, Ewald Hering, that it is innate. However you lean on this issue, the point is that while perceptual constancy was not discussed by Moore, Carnap, or Quine, philosophers of perception have come to realize that some account needs to be given.
Cross-modal, sensorimotor processes of perceptual exploration are a new agenda item for epistemology and philosophy of perception. How did I integrate the information that comes to me by the temporally extended process that I described earlier? Some of the processing that I employ here is innate, some naturally emerge (in the way that language does) with normal exposure; still others are learned or are taught. How does location by tactile exploration coordinate with location by sight? Recent work by Pawan Sinha of MIT on genital cataract patients in India—they see for the very first time in adulthood—demonstrates that this ability is innate. Immediately on waking up from surgery the patients know how to look at what they are touching. On the other hand, shape identification takes time, though only a few days—likely too short for associative learning. No doubt, there are other visual skills that take longer, and some, perhaps, that have to be taught.
Putting the Helmholtzian question to one side again—the question of innateness vs learning—there is a question about the epistemology of perceptual exploration. Was the knowledge that I obtained about the stiff thread in my jacket was direct, or in other words, available to me independently of other knowledge gained during the process. Did I just come to know what was causing the problem, or did I first have to come to know other things (for example, the change in the sensation with changes of my bodily position) that allowed me to infer the cause of irritation and its location?
What is clear, one way or another, is that perceptual knowledge is gained by an interactive process that involves the body. How does this sit with recent sensorimotor theories? In my opinion, the sensorimotor theories address these phenomena in the wrong way. Here is a simple example that demonstrates my worry. At a traffic light, red means stop and green means go. How do you see a stopping light? You see it as red, do you not. Similarly, somebody throws a Frisbee in your direction. As it spins toward you, it presents different shape-projections to your eye. What do you see? A rigid disc—though it may be somewhat indeterminate whether it is really a disc or more like an ellipse, it is absolutely clear that it is not changing in shape. Sensorimotor theorists are generally in conflict with these simple truths. They say that what you see somehow reflects your engagement with the world. Thus, they might say that you see the light as a stopping light, or that you see the disc as a sensorimotor “profile”, i.e., as a series of projected shapes.
To my way of thinking, this is the wrong way to address the involvement of bodily exploration. Bodily exploration is meant to get at how things are independently of the perceiver’s perspective and goals. In my little tale of exploration above, the point of all of the motion was to obtain many different perspectives on the world. Once you have many perspectives, you can “solve for” the view from nowhere.