What is it like to be a bat?
A classic, relatively short paper by Thomas Nagel. I’ve searched before, but couldn’t find it online until now.
Nagel argues that the essential feature of consciousness is that there is something it’s like to be conscious. A bat is conscious, so there’s something it’s like for a bat to be a bat. Bats are sufficiently similar to humans that we can be pretty sure they’re conscious — almost as certain as we can be that other humans are conscious — but they’re also very different from us. They navigate via sonar, which is a sense that humans simply don’t have. We have no idea what it’s like for a bat to perceive the world in sonar; at best, we can only imagine what it’d be like for us to do so:
To the extent that I could look and behave like a wasp or a bat without changing my fundamental structure, my experiences would not be anything like the experiences of those animals. On the other hand, it is doubtful that any meaning can be attached to the supposition that I should possess the internal neurophysiological constitution of a bat. Even if I could by gradual degrees be transformed into a bat, nothing in my present constitution enables me to imagine what the experiences of such a future stage of myself thus metamorphosed would be like. The best evidence would come from the experiences of bats, if we only knew what they were like.
A few paragraphs later, we come to the crux of Nagel’s argument:
This bears directly on the mind-body problem. For if the facts of experience—facts about what it is like for the experiencing organism—are accessible only from one point of view, then it is a mystery how the true character of experiences could be revealed in the physical operation of that organism. The latter is a domain of objective facts par excellence—the kind that can be observed and understood from many points of view and by individuals with differing perceptual systems. There are no comparable imaginative obstacles to the acquisition of knowledge about bat neurophysiology by human scientists, and intelligent bats or Martians might learn more about the human brain than we ever will. (…)
In the case of experience, on the other hand, the connection with a particular point of view seems much closer. It is difficult to understand what could be meant by the objective character of an experience, apart from the particular point of view from which its subject apprehends it. After all, what would be left of what it was like to be a bat if one removed the viewpoint of the bat? But if experience does not have, in addition to its subjective character, an objective nature that can be apprehended from many different points of view, then how can it be supposed that a Martian investigating my brain might be observing physical processes which were my mental processes (as he might observe physical processes which were bolts of lightning), only from a different point of view? How, for that matter, could a human physiologist observe them from another point of view?
In other words, how can we reduce consciousness to the physical, objective qualities of the brain, if the essential characteristic of consciousness — the what-it’s-like-for-the-organism characteristic — is only accessible from a given, subjective point of view, and the only way of being objective is to step out of this point of view? Nagel is arguing that objectivity can only be achieved by shedding subjectivity, but if there’s nothing left of subjectivity once we have cast it off and become objective, how can that be an explanation of the subjective? If the essential characteristic of consciousness — again, the feeling of what it’s like to be something conscious — is incomprehensible in terms of the objective, how then can the objective ever explain consciousness?
Later on, Nagel says this:
At the present time the status of physicalism is similar to that which the hypothesis that matter is energy would have had if uttered by a pre-Socratic philosopher. We do not have the beginnings of a conception of how it might be true. In order to understand the hypothesis that a mental event is a physical event, we require more than an understanding of the word ‘is’. The idea of how a mental and a physical term might refer to the same thing is lacking, and the usual analogies with theoretical identification in other fields fail to supply it. They fail because if we construe the reference of mental terms to physical events on the usual model, we either get a reappearance of separate subjective events as the effects through which mental reference to physical events is secured, or else we get a false account of how mental terms refer (for example, a causal behaviorist one).
So what he’s saying is that not only do current physicalist theories fail to explain consciousness, but we don’t even have a conception of what it would be like for them to be true. We simply don’t know how they could possibly be true — which is different from knowing that they couldn’t be true.
I don’t pretend to know how the physical gives rise to the mental. I’m convinced that it does because there’s no evidence to suggest there is anything other than the physical — and whatever explains consciousness, it must be something that exists — and because every other proposed explanation is equally mysterious. We have no conception of how something non-physical could possibly explain consciousness either. Simply making up a non-physical substance and calling it “mind” doesn’t explain anything; this new substance would fall prey to the exact same puzzles that matter does. And if you suggest that mind is not, and isn’t caused by any substance at all, a pertinent question is how it could exist at all. Helpfully, Nagel provides us with a good analogy that shows how, by the process of elimination, it seems reasonable to believe that consciousness is caused by the physical, even if we have no idea how or even a concept of what an explanation would look like:
Strangely enough, we may have evidence for the truth of something we cannot really understand. Suppose a caterpillar is locked in a sterile safe by someone unfamiliar with insect metamorphosis, and weeks later the safe is reopened, revealing a butterfly. If the person knows that the safe has been shut the whole time, he has reason to believe that the butterfly is or was once the caterpillar, without having any idea in what sense this might be so. (One possibility is that the caterpillar contained a tiny winged parasite that devoured it and grew into the butterfly.)
It is conceivable that we are in such a position with regard to physicalism.
It’s often said that correlation doesn’t equal causation, which is true, but what’s seldom said is that the only way we have of establishing causation is by extrapolating from correlation. Two billiard balls smack against each other and change direction; we cannot sense some kind of mysterious “causal power” here, we can only observe the correlation, confirm it by repeated observation, and form theories of how it might work. Neurology has this epistemological restriction, like every other science. The only way we can say that certain patterns of neuron firings give rise to certain subjective experiences is by observing correlations.
The problem is that we can give a physical explanation of why the balls must change direction when they hit each other; these explanations give rise to testable predictions, and by confirming the predictions, we can see that it would contradict physical laws if the balls behaved differently. No one has been able to formulate any physical explanations of why certain patterns of neurons firing must give rise to certain subjective experiences, or else be inconsistent with physical law. This is the problem of philosophical zombies: we can certainly imagine zombies that are physically identical to humans, but aren’t conscious. As Nagel says in a footnote: “Perhaps there could not actually be such robots. Perhaps anything complex enough to behave like a person would have experiences.” But that cannot be assumed, it must be proved, and no one has managed to prove it yet.
In another footnote, Nagel writes:
The problem is not just that when I look at the Mona Lisa, my visual experience has a certain quality, no trace of which is to be found by someone looking into my brain. For even if he did observe there a tiny image of the Mona Lisa, he would have no reason to identify it with the experience.
I disagree that he would have no reason to associate the tiny image with the experience; as I’ve argued, correlation is our only way of proving causation, and if the tiny image showed a high correlation with the experience, we’d have every reason to suspect that the two are connected (even if they might not be identical). But the problem is not just explaining that this is so, it’s explaining why: not just that it happens to be the case that certain neurons firing in such-and-such a way cause certain subjective experiences, the problem is giving an explanation such that it would contradict physical laws if the neurons did not cause the given experience. As it stands, it seems perfectly consistent with physics that human brains are unconscious.
On the other hand, when someone peers into a brain, a web of neurons firing, and asks “where is the consciousness?”, an image that comes to mind is that of a physicist — a supernaturally equipped physicist who can track every particle completely — who maps out the paths of every particle in a game of football. Every particle that makes up the field, the ball, the players, the stadium, the supporters, the air, everything. Let’s say it’s a game of football played and cheered by robots, so we don’t run into any kind of subjectivity at all. It seems to me that the physicist might easily ask, looking at the data: “where’s the game of football?” And I can’t make the analogy rigourous, but that’s sort of what I feel a philosopher looking at a web of neurons and failing to locate consciousness is doing.
I wonder if there are certain facts that are simply incomprehensible to humans, and if the relationship between mind and matter is one of them. In Division by Zero, a short story by the science fiction writer Ted Chiang — an author I love, by the way — a mathematician discovers that elementary arithmetic is inconsistent. In other words, what seems to us to be obvious actually contains a contradiction. What if it is? What if what seems to us to be built into logic — maybe even something as basic as modus ponens — is actually invalid? What if our belief that something is inherently logical is simply an artifact of our neurology? This gives rise to a novel kind of skepticism: imagine a race of humans that think that a logically invalid argument is valid. To them, it seems perfectly logical that the argument is valid. They feel it’s as obviously logical as modus ponens or 1+1 = 2 feels to us. What would be the difference between what it’s like for us to be us and what it’s like to be them for them? Maybe it would feel exactly the same to them when they consider this invalid argument valid as it feels to us when we consider 1 + 1 = 2 or modus ponens to be valid. And if we can imagine these humans, how do we know we’re not them? If there’s nothing in our experience that could differentiate between our feeling like an invalid principle is obviously logical, and our feeling that a valid principle is obviously logical, how can we be sure that any logical principle is logical?
Conversely, maybe there are facts that are perfectly logical, but that we cannot comprehend, because of the way our psychology is made up. Maybe there are facts that we could immediately discover, if only our psychologies were capable of comprehending them. Maybe the relationship between mind and matter is one such fact. Maybe the way consciousness arises from the physical is perfectly logical, but still as incomprehensible to us as what it’s like to be a bat is.
