This week, I’m blogging about my new book, The Epistemic Role of Consciousness (Oxford University Press, September 2019). Today, I’ll discuss the epistemic role of consciousness in perception.
Human perception is normally conscious: there is something it is like for us to perceive the world around us. And yet there are unconscious analogues of perception in which our perceptual systems represent information about the external world in way that makes no impact on conscious experience. For example, patients with blindsight have large blind spots caused by neural damage in the primary visual cortex. When a stimulus is presented in the blind spot, many of these subjects do not experience it at all. But when they are forced to guess what it is, e.g. an ‘X’ or an ‘O’, they can often give the right answer with a high degree of reliability. What explains this remarkable ability is the unconscious visual representation of the stimulus in a subcortical region of the brain.
Can patients with blindsight acquire knowledge about the blind spot on the basis of unconscious visual information? Over time, they can learn of their own reliability and so come to know that their guesses are probably true. But we tend to think that our perceptual knowledge of the environment is rather more immediate than this. For instance, you can know just by looking whether the lights are on right now and you don’t need to infer this conclusion from premises about your own reliability. Given this point, a better question to ask is whether patients with blindsight can acquire non-inferential knowledge about the blind spot on the basis of unconscious visual information. In Chapter 3: Perception, I argue that the answer is no.
Consider an analogy. Suppose you’re in hospital under a general anesthetic. While you’re sleeping, a neuroscientist implants a mechanism in your brain that registers information about objects outside your conscious field of vision. When you wake up, the doctor holds her hand behind your head and asks you to guess how many fingers she is holding up. To your surprise, the implanted mechanism enables you to reliably guess the right answer. Can you know the right answer without inference? Presumably not, since you have no justification to believe that one answer is more likely to be true than any other. Under the circumstances, the only justified attitude is to withhold belief.
I’m inclined to say exactly the same in the case of blindsight. In case you’re not persuaded, however, here’s an argument designed to bring you around. The first premise is the empirical observation that patients with blindsight tend to withhold from forming beliefs about the blind spot until they have learned about their own reliability. They can be prompted to guess, but they are surprised to learn that their guesses are reliable. The second premise is that the normative observation that this reaction seems entirely rational. Patients with blindsight are no less rational than the rest of us: theirs is a perceptual deficit, rather than a cognitive one. The third premise is that the epistemic principle that if unconscious perceptual information in blindsight provides evidence that justifies beliefs about the blind spot, then withholding belief about the blind spot is less than fully rational. After all, epistemic rationality requires conforming your beliefs to the evidence. From these three premises, I conclude that unconscious perceptual information in blindsight doesn’t provide evidence that justifies beliefs about the blind field after all.
If perceptual experience justifies belief about the external world, then what’s missing in blindsight? It’s not enough to point to causal differences between perception and blindsight, since these differences can be stipulated away. As we’ve seen, patients with blindsight don’t typically form beliefs about the blind spot without relying on premises about their own reliability. But we can easily imagine a patient with super-blindsight who spontaneously forms beliefs about the blind spot on the basis of unconscious perceptual information. Still, we can ask, are these beliefs justified? And the answer seems no different from before. Withholding belief is still the only rational reaction. The mere fact that a subject is disposed to form beliefs is not enough to make them rational. Indeed, our patient with super-blindsighter seems irrational to the extent that he is disposed to form beliefs about the blind spot without inference.
What’s missing in blindsight, I argue, is phenomenal consciousness. Perception justifies belief about the external world if and only if it is phenomenally conscious. Moreover, perception justifies belief in virtue of its phenomenal character alone. It is solely because of the phenomenal character of perceptual experience that it justifies belief without standing in need of any justification. One consequence of this claim is that we can avoid the new evil demon problem: your perceptual experience justifies beliefs about the external world even when you are deceived by a Cartesian evil demon (Cohen 1984).
Why must perception be phenomenally conscious in order to justify beliefs about the external world? I address this question in the second part of the book, but here is the short answer. Perceptual experience does a kind of double epistemic duty: it not only justifies beliefs about the external world, but it also justifies beliefs about perceptual experience. More specifically, perceptual experience justifies beliefs about the phenomenal features in virtue of which it justifies beliefs about the external world. When you have a perceptual experience in which it seems to you that p in the absence of defeaters, you thereby have justification to believe:
- It seems to me that p in the absence of defeaters [by introspection].
- If it seems to me that p in the absence of defeaters, then I have epistemic justification to believe that p [by a priori reasoning].
- Therefore, I have epistemic justification to believe that p [by deduction].
This means that your perceptual experience cannot justify believing that p without also justifying the belief that you have justification to believe that p.
In contrast, unconscious perceptual information in blindsight cannot play the same dual role as perceptual experience, since it doesn’t justify beliefs about itself. It has no phenomenal features that seem upon reflection to justify beliefs about the external world. If we suppose that blindsight can justify the belief that p, without also justifying the belief that it justifies believing that p, then we must be prepared to countenance scenarios in which you have justification to believe abominable conjunctions of the following forms:
- p and I don’t have justification to believe that p; or
- p and it’s an open question whether I have justification to believe that p.
I argue, however, that you can never have justification to believe abominable conjunctions of these forms. This bolsters the arguments in Chapter 3 that perception must be phenomenally conscious in order to justify beliefs about the external world. What does this mean for the mental lives of zombies? Zombies, like the zombie systems within us, can represent information about the world around us. But zombies cannot acquire knowledge, or epistemically justified belief, on the basis of their unconscious mental representations. And yet perception is a source of knowledge under the right external conditions. Hence, zombies do not perceive the world. If you like, we can say they perzieve the world. But perzeption is no substitute for perception. Although it can help you to navigate the world, it cannot give you knowledge of the external world.