Hume was skeptical of induction and causality. Descartes began his philosophy by doubting everything. Both thought we may be in great error about the external world. But neither could bring themselves to seriously doubt the contents of their own subjective conscious experience.
Philosophers and non-philosophers alike often say: "I may not know whether that is really a yellow banana before me, but surely I know the character of my visual experience of a yellow banana! I may not know whether I really just dropped a barbell on my toe, but surely I know the subjective character of my pain experience, right?"
In this article I hope to persuade you that yes, you can be wrong about the subjective quality of your own conscious experience. In fact, such errors are common.
Thomas Nagel famously said that we cannot imagine the subjective experience of bat sonar:
Bat sonar, though clearly a form of perception, is not similar in its operation to any sense that we possess, and there is no reason to suppose that it is subjectively like anything we can experience or imagine.1
Hold up a book in front of your face at arm's length, close your eyes, and say something loudly. Can you hear the emptiness of the space in front of you? Close your eyes again, hold the book directly in front of your face, and say the book's name again. Can you now hear that the book is closer?
I'll bet you can, and thus you may be more bat-like than Nagel seems to think is possible, and more bat-like than you have previously thought. When I discovered this, I realized that not only had I been wrong about my perceptual capabilities, I had also been ignorant of the daily content of my subjective auditory experience.
Blind people can be especially good at using echolocation to navigate the world. Just like bats and dolphins and whales (but less accurately), humans can make sounds and then hear how nearby objects reflect and modify those sounds. People with normal vision can also be trained to echolocate to some degree with training, for example detecting the location of walls while blindfolded.2 After some practice, blindfolded people can use sound to distinguish objects of different shapes and textures (at a rate significantly better than chance).3
You can try this yourself. Get a friend to blindfold you and then move their hand to one of four quadrants of space in front of your face. Try hissing or talking loudly and see if you can tell something about where your friend's hand is. Have your friend move their hand to another quadrant, and try again. Do this a few dozen times. I suspect you will find that after a while you'll do better than chance at locating the quadrant your friend's hand is in, and you may be able to tell something about its distance as well. If so, you are echolocating. You are having an auditory experience of the physical location of an object - something you may not have realized that you can do, something you probably have been doing your whole life without much realizing it.
Alternatively, have a friend blindfold you and place you some unspecified distance from a wall. Step toward the wall a few inches at a time, speaking loudly, and stop when the wall is directly in front of you. Most people find they can do this quite reliably. But of course you can't see or touch the wall, and the wall is making no sound of its own. You are echolocating.
One final test to prove it to yourself, this one relevant to shape and texture. Close your eyes, repeat some syllable, and have a friend hold one of three objects in front of your face: a book, a wadded-up T-shirt, and a mixing bowl. I think you'll find that you can distinguish between these three silent objects better than chance, and that the book will sound solid, the T-shirt will sound soft, and the mixing bowl will sound hollow. You are echolocating shape and texture.
Does a coin look circular?
Set a coin on a desk or table three feet in front of you. Is its shape circular or elliptical?
One popular view is to say that we perceive the coin in two aspects. In one aspect, we perceive the raw sense data from our visual plane, which shows the coin as being elliptical (because one end of it is stretching away from us). In another aspect, we perceive it as circular because our minds have an intuitive physics about the shape permanence of solid objects. Perhaps our minds 'flip' between seeing the coin from the two aspects - a kind of 'Gestalt shift' - just as it flips between seeing a rabbit and a duck in Wittgenstein's famous duck-rabbit drawing.
Schwitzgebel (2011) reports his own confusion:
What exactly is my sensory experience as I stare at a penny? My first and recurring inclination is to say that the penny looks just plain circular, in a three-dimensional space - not elliptical at all... However, I also find that if I dip my head lower to view the penny from a flatter angle, I begin to see how one might think it looks elliptical. Closing one eye helps too... Am I experiencing the ellipse too? Maybe not. But neither can I say that I noticed any Gestalt shift... Could it be, simply, that my visual experience is disorganized, so that there is no simple relationship between viewing angle and apparent shape...?
Maybe my terms and concepts are muddled. What is it for something to 'look elliptical'? ...
Or am I simply a poor introspector? Maybe the fact that my own phenomenology in this case doesn't seem obvious to me reveals my introspective ineptitude... And yet I am not sure I should trust other [people's] introspections either.4
The character of our subjective experience of shape at varying angles and distances is widely debated by philosophers and psychologists,4 lending some support to the claim that we are unsure of it.
What is the character of an imagined scene?
Close your eyes and picture the front of your house or apartment building from the street.
Presumably, you know that you experience an image, and you know some aspects of its content (that it is a house, from a certain angle). But what else do you know? Schwitzgebel questions:
How much of the scene can you vividly visualize at once? Can you keep the image of the chimney vividly in mind at the same time that you vividly imagine your front door, or how does the image of the chimney fade as you begin to think about the door? How much detail does your image have? How stable is it? If you can't visually imagine the entire front of your house in rich detail all at once, what happens to the aspects of the image that are relatively less detailed? If the chimney is still experienced as part of the imagery when your imagemaking energies are focused on the front door, how exactly is it experienced? Does it have determinate shape, determinate color? In general, do the objects in your image have color before you think to assign color to them, or do some of the colors remain indeterminate, at least for a while...? If there is indeterminacy of color, how is that indeterminacy experienced? As gray? Does your visual image have depth in the same way that your sensory experienced does... or is your imagery somehow flatter...? ...Do you experience the image as located somewhere in egocentric space - inside your head, or before your eyes, or in front of your forehead - or does it make no sense to attempt to assign it a position in this way?5
When questioned in this way, most people quickly become uncertain about the character of their own subjective conscious experience.
Do you dream in color?
Most people, when asked, are fairly confident of their answer. But the answer given (in questionnaires or after awakened during REM sleep) has varied widely throughout history and between persons.6 Pre-scientific authors tended to assume they dreamed in color, while studies in the first half of the 20th century found very few people who reported dreaming in color. In the 1960s, this consensus was overturned, and recent studies show that today, more than 80% of people report that they dream in color. But there are also certain populations that overwhelming report dreaming in black and white.
Is there something in our genes or in the air that decides whether or not we dream in color, or are we confused about our own subjective experience?
Schwitzgebel reviews the arguments back and forth, but none give a clear answer. And unfortunately, there remains much disagreement about the neurology of color experience.7
Is experience persistent?
Do you have constant tactile experience of your feet in your shoes?... Constant visual experience of the frames of your eyeglasses? ... Is consciousness abundant, the stream of experience bristling with phenomenology in a wide variety of modalities simultaneously (visual, auditory, tactile, olfactory, imagistic, proprioceptive, emotional), or is it sparse, limited to one or a few things at a time?
Suppose you have driven to work by the same route a thousand times. Today, you are absorbed in remembering an unpleasant interaction with your department head. Traffic is light, no dangerous situation occurs, and you drive habitually. You arrive at your usual parking area and seem to "wake up" - "Ah, I'm at work already!" - with virtually no memory of having driven there. Did you have visual experience while you were driving? You responded to events on the road, stopped at red lights, and stayed in your lane... But perhaps visual input can influence behavior without the involvement of consciousness.
These are difficult questions, and both experts and laymen disagree as to their answers.8
Sound and vision biases
Have you noticed that you perceive vertical distance as greater when you are looking down than when you are looking up? Well, you do.9
Have you noticed that you perceive changes in 'approaching' sounds as being greater than equivalent changes in 'receding' sounds? Have you noticed that you perceived 'approaching' sounds as occurring more near to you than 'receding' sounds? You do.10
As many people who practices meditation seriously can tell you, the subjective contents of conscious experience are often surprising and uncertain. We can be wrong about our own subjective conscious experiences. Thus, they cannot serve as a bedrock for certainty and a priori truth. (At least, minimally complex subjective conscious experiences cannot.)
It seems that all we can do is exercise some naturalized epistemology: "reflecting on your mind's degree of trustworthiness, using your current mind as opposed to something else." Luckily, the brain is the lens that sees its flaws, as demonstrated by surprising studies in human echolocation, the color of dreams, and more.
1 Nagel (1974).
2 Supa et al. (1944); Ammons et al. (1953); Roseblum et al. (2000).
3 Hausfeld et al. (1982); Rosenblum & Robert (2007); Gordon & Rosenblum (2004).
4 Schwitzgebel (2011), chapter 2. Note that I also interviewed the author here. This post is basically a summary of a few sections of Schwitzgebel's book.
5 Schwitzgebel (2011), chapter 3.
6 Schwitzgebel (2011) reports that most people he ask about dreaming in color are fairly confident of their answer. Also see his Table 1.1, which summarizes the results of 21 studies on the reported color of dreams, some of which include confidence measures. Chapter 1 contains a summary of historical assumptions about dreaming in color.
7 Gegenfurtner & Kiper (2003); Solomon & Lennie (2007); Wade et al. (2008); Conway (2009).
8 Schwitzgebel (2011), chapter 6.
9 Jackson & Cormack (2008).
10 Neuhoff (2001).
Ammons, Worchel, & Dallenbach (1953). Facial vision: The perception of obstacles out of doors by blindfolded and blindfolded-deafened subjects. American Journal of Psychology, 66: 519-554.
Conway (2009). Color vision, cones, and color-coding in the cortex. Neuroscientist, 15: 274-290.
Gegenfurtner & Kiper (2003). Color vision. Annual Review of Neuroscience, 26: 181-206.
Gordon & Rosenblum (2004). Perception of sound-obstructing surfaces using body-scaled judgments. Ecological Psychology, 16: 87-113.
Hausfeld, Power, Gorta, & Harris (1982). Echo perception of shape and texture by sighted subjects. Perceptual and Motor Skills, 55: 623-632.
Jackson & Cormack (2008). Evolved navigation theory and the descent illusion. Evolution and Human Behavior, 29: 299-304.
Nagel (1974). What is it like to be a bat? Philosophical Review, 83: 435-450.
Neuhoff (2001). An adaptive bias in the perception of looming auditory motion. Ecological Psychology, 13: 87-113.
Roseblum, Gordon, & Jarquin (2000). Echolocating distance by moving and stationary listeners. Ecological Psychology, 12: 181-206.
Rosenblum & Robert (2007). Hearing silent shapes: Identifying the shape of a sound-obstructing surface. Ecological Psychology, 19: 351-366.
Schwitzgebel (2011). Perplexities of Consciousness. MIT Press.
Solomon & Lennie (2007). The machinery of colour vision. Nature Reviews Neuroscience, 8: 276-286.
Supa, Cotzin, & Dallenbach (1944). Facial vision: the perception of obstacles by the blind. ASmerican Journal of Psychology, 62: 133-183.
Wade, Augath, Logothetis, & Wandell (2008). fMRI measurement of color in macaque and human. Journal of Vision, 8(10): 1-19.