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What is it like to be a bat

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What is it like to be a bat?

Thomas Nagel

[From The Philosophical Review LXXXIII, 4 (October 1974):

435-50.]

Consciousness is what makes the mind-body problem really intractable. Perhaps that is why current

discussions of the problem give it little attention or get it obviously wrong. The recent wave of

reductionist euphoria has produced several analyses of mental phenomena and mental concepts designed

to explain the possibility of some variety of materialism, psychophysical identification, or reduction. 1 But

the problems dealt with are those common to this type of reduction and other types, and what makes the

mind-body problem unique, and unlike the water-H 2 O problem or the Turing machine-IBM machine

problem or the lightning-electrical discharge problem or the gene-DNA problem or the oak

tree-hydrocarbon problem, is ignored.

Every reductionist has his favorite analogy from modern science. It is most unlikely that any of these

unrelated examples of successful reduction will shed light on the relation of mind to brain. But

philosophers share the general human weakness for explanations of what is incomprehensible in terms

suited for what is familiar and well understood, though entirely different. This has led to the acceptance of

implausible accounts of the mental largely because they would permit familiar kinds of reduction. I shall

try to explain why the usual examples do not help us to understand the relation between mind and

body—why, indeed, we have at present no conception of what an explanation of the physical nature of a

mental phenomenon would be. Without consciousness the mind-body problem would be much less

interesting. With consciousness it seems hopeless. The most important and characteristic feature of

conscious mental phenomena is very poorly understood. Most reductionist theories do not even try to

explain it. And careful examination will show that no currently available concept of reduction is

applicable to it. Perhaps a new theoretical form can be devised for the purpose, but such a solution, if it

exists, lies in the distant intellectual future.

Conscious experience is a widespread phenomenon. It occurs at many levels of animal life, though we

cannot be sure of its presence in the simpler organisms, and it is very difficult to say in general what

provides evidence of it. (Some extremists have been prepared to deny it even of mammals other than

man.) No doubt it occurs in countless forms totally unimaginable to us, on other planets in other solar

systems throughout the universe. But no matter how the form may vary, the fact that an organism has

conscious experience at all means, basically, that there is something it is like to be that organism. There

may be further implications about the form of the experience; there may even (though I doubt it) be

implications about the behavior of the organism. But fundamentally an organism has conscious mental

states if and only if there is something that it is to be that organism—something it is like for the

organism.

We may call this the subjective character of experience. It is not captured by any of the familiar, recently

devised reductive analyses of the mental, for all of them are logically compatible with its absence. It is not

analyzable in terms of any explanatory system of functional states, or intentional states, since these could

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be ascribed to robots or automata that behaved like people though they experienced nothing. 2 It is not

analyzable in terms of the causal role of experiences in relation to typical human behavior—for similar

reasons. 3 I do not deny that conscious mental states and events cause behavior, nor that they may be given

functional characterizations. I deny only that this kind of thing exhausts their analysis. Any reductionist

program has to be based on an analysis of what is to be reduced. If the analysis leaves something out, the

problem will be falsely posed. It is useless to base the defense of materialism on any analysis of mental

phenomena that fails to deal explicitly with their subjective character. For there is no reason to suppose

that a reduction which seems plausible when no attempt is made to account for consciousness can be

extended to include consciousness. With out some idea, therefore, of what the subjective character of

experience is, we cannot know what is required of physicalist theory.

While an account of the physical basis of mind must explain many things, this appears to be the most

difficult. It is impossible to exclude the phenomenological features of experience from a reduction in the

same way that one excludes the phenomenal features of an ordinary substance from a physical or chemical

reduction of it—namely, by explaining them as effects on the minds of human observers. 4 If physicalism

is to be defended, the phenomenological features must themselves be given a physical account. But when

we examine their subjective character it seems that such a result is impossible. The reason is that every

subjective phenomenon is essentially connected with a single point of view, and it seems inevitable that

an objective, physical theory will abandon that point of view.

Let me first try to state the issue somewhat more fully than by referring to the relation between the

subjective and the objective, or between the pour-soi and the en-soi . This is far from easy. Facts about

what it is like to be an X are very peculiar, so peculiar that some may be inclined to doubt their reality, or

the significance of claims about them. To illustrate the connection between subjectivity and a point of

view, and to make evident the importance of subjective features, it will help to explore the matter in

relation to an example that brings out clearly the divergence between the two types of conception,

subjective and objective.

I assume we all believe that bats have experience. After all, they are mammals, and there is no more doubt

that they have experience than that mice or pigeons or whales have experience. I have chosen bats instead

of wasps or flounders because if one travels too far down the phylogenetic tree, people gradually shed

their faith that there is experience there at all. Bats, although more closely related to us than those other

species, nevertheless present a range of activity and a sensory apparatus so different from ours that the

problem I want to pose is exceptionally vivid (though it certainly could be raised with other species). Even

without the benefit of philosophical reflection, anyone who has spent some time in an enclosed space with

an excited bat knows what it is to encounter a fundamentally alien form of life.

I have said that the essence of the belief that bats have experience is that there is something that it is like

to be a bat. Now we know that most bats (the microchiroptera, to be precise) perceive the external world

primarily by sonar, or echolocation, detecting the reflections, from objects within range, of their own

rapid, subtly modulated, high-frequency shrieks. Their brains are designed to correlate the outgoing

impulses with the subsequent echoes, and the information thus acquired enables bats to make precise

discriminations of distance, size, shape, motion, and texture comparable to those we make by vision. But

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. This

appears to create difficulties for the notion of what it is like to be a bat. We must consider whether any

method will permit us to extrapolate to the inner life of the bat from our own case, 5 and if not, what

alternative methods there may be for understanding the notion.

Our own experience provides the basic material for our imagination, whose range is therefore limited. It

will not help to try to imagine that one has webbing on one's arms, which enables one to fly around at

dusk and dawn catching insects in one's mouth; that one has very poor vision, and perceives the

surrounding world by a system of reflected high-frequency sound signals; and that one spends the day

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hanging upside down by one's feet in an attic. In so far as I can imagine this (which is not very far), it tells

me only what it would be like for me to behave as a bat behaves. But that is not the question. I want to

know what it is like for a bat to be a bat. Yet if I try to imagine this, I am restricted to the resources of my

own mind, and those resources are inadequate to the task. I cannot perform it either by imagining

additions to my present experience, or by imagining segments gradually subtracted from it, or by

imagining some combination of additions, subtractions, and modifications.

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.

So if extrapolation from our own case is involved in the idea of what it is like to be a bat, the

extrapolation must be incompletable. We cannot form more than a schematic conception of what it is like.

For example, we may ascribe general types of experience on the basis of the animal's structure and

behavior. Thus we describe bat sonar as a form of three-dimensional forward perception; we believe that

bats feel some versions of pain, fear, hunger, and lust, and that they have other, more familiar types of

perception besides sonar. But we believe that these experiences also have in each case a specific

subjective character, which it is beyond our ability to conceive. And if there's conscious life elsewhere in

the universe, it is likely that some of it will not be describable even in the most general experiential terms

available to us. 6 (The problem is not confined to exotic cases, however, for it exists between one person

and another. The subjective character of the experience of a person deaf and blind from birth is not

accessible to me, for example, nor presumably is mine to him. This does not prevent us each from

believing that the other's experience has such a subjective character.)

If anyone is inclined to deny that we can believe in the existence of facts like this whose exact nature we

cannot possibly conceive, he should reflect that in contemplating the bats we are in much the same

position that intelligent bats or Martians 7 would occupy if they tried to form a conception of what it was

like to be us. The structure of their own minds might make it impossible for them to succeed, but we

know they would be wrong to conclude that there is not anything precise that it is like to be us: that only

certain general types of mental state could be ascribed to us (perhaps perception and appetite would be

concepts common to us both; perhaps not). We know they would be wrong to draw such a skeptical

conclusion because we know what it is like to be us. And we know that while it includes an enormous

amount of variation and complexity, and while we do not possess the vocabulary to describe it adequately,

its subjective character is highly specific, and in some respects describable in terms that can be understood

only by creatures like us. The fact that we cannot expect ever to accommodate in our language a detailed

description of Martian or bat phenomenology should not lead us to dismiss as meaningless the claim that

bats and Martians have experiences fully comparable in richness of detail to our own. It would be fine if

someone were to develop concepts and a theory that enabled us to think about those things; but such an

understanding may be permanently denied to us by the limits of our nature. And to deny the reality or

logical significance of what we can never describe or understand is the crudest form of cognitive

dissonance.

This brings us to the edge of a topic that requires much more discussion than I can give it here: namely,

the relation between facts on the one hand and conceptual schemes or systems of representation on the

other. My realism about the subjective domain in all its forms implies a belief in the existence of facts

beyond the reach of human concepts. Certainly it is possible for a human being to believe that there are

facts which humans never will possess the requisite concepts to represent or comprehend. Indeed, it would

be foolish to doubt this, given the finiteness of humanity's expectations. After all there would have been

transfinite numbers even if everyone had been wiped out by the Black Death before Cantor discovered

them. But one might also believe that there are facts which could not ever be represented or

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comprehended by human beings, even if the species lasted for ever—simply because our structure does

not permit us to operate with concepts of the requisite type. This impossibility might even be observed by

other beings, but it is not clear that the existence of such beings, or the possibility of their existence, is a

precondition of the significance of the hypothesis that there are humanly inaccessible facts. (After all, the

nature of beings with access to humanly inaccessible facts is presumably itself a humanly inaccessible

fact.) Reflection on what it is like to be a bat seems to lead us, therefore, to the conclusion that there are

facts that do not consist in the truth of propositions expressible in a human language. We can be

compelled to recognize the existence of such facts without being able to state or comprehend them.

I shall not pursue this subject, however. Its bearing on the topic before us (namely, the mind-body

problem) is that it enables us to make a general observation about the subjective character of experience.

Whatever may be the status of facts about what it is like to be a human being, or a bat, or a Martian, these

appear to be facts that embody a particular point of view.

I am not adverting here to the alleged privacy of experience to its possessor. The point of view in question

is not one accessible only to a single individual. Rather it is a type. It is often possible to take up a point of

view other than one's own, so the comprehension of such facts is not limited to one's own case. There is a

sense in which phenomenological facts are perfectly objective: one person can know or say of another

what the quality of the other's experience is. They are subjective, however, in the sense that even this

objective ascription of experience is possible only for someone sufficiently similar to the object of

ascription to be able to adopt his point of view—to understand the ascription in the first person as well as

in the third, so to speak. The more different from oneself the other experiencer is, the less success one can

expect with this enterprise. In our own case we occupy the relevant point of view, but we will have as

much difficulty understanding our own experience properly if we approach it from another point of view

as we would if we tried to understand the experience of another species without taking up its point of

view. 8

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.

This is not by itself an argument against reduction. A Martian scientist with no understanding of visual

perception could understand the rainbow, or lightning, or clouds as physical phenomena, though he would

never be able to understand the human concepts of rainbow, lightning, or cloud, or the place these things

occupy in our phenomenal world. The objective nature of the things picked out by these concepts could be

apprehended by him because, although the concepts themselves are connected with a particular point of

view and a particular visual phenomenology, the things apprehended from that point of view are not: they

are observable-from the point of view but external to it; hence they can be comprehended from other

points of view also, either by the same organisms or by others. Lightning has an objective character that is

not exhausted by its visual appearance, and this can be investigated by a Martian without vision. To be

precise, it has a more objective character than is revealed in its visual appearance. In speaking of the move

from subjective to objective characterization, I wish to remain noncommittal about the existence of an end

point, the completely objective intrinsic nature of the thing, which one might or might not be able to

reach. It may be more accurate to think of objectivity as a direction in which the understanding can travel.

And in understanding a phenomenon like lightning, it is legitimate to go as far away as one can from a

strictly human viewpoint. 9

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

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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? 10

We appear to be faced with a general difficulty about psychophysical reduction. In other areas the process

of reduction is a move in the direction of greater objectivity, toward a more, accurate view of the real

nature of things. This is accomplished by reducing our dependence on individual or species-specific points

of view toward the object of investigation. We describe it not in terms of the impressions it makes on our

senses, but in terms of its more general effects and of properties detectable by means other than the human

senses. The less it depends on a specifically human viewpoint, the more objective is our description. It is

possible to follow this path because although the concepts and ideas we employ in thinking about the

external world are initially applied from a point of view that involves our perceptual apparatus, they are

used by us to refer to things beyond themselves—toward which we have the phenomenal point of view.

Therefore we can abandon it in favor of another, and still be thinking about the same things.

Experience itself however, does not seem to fit the pattern. The idea of moving from appearance to reality

seems to make no sense here. What is the analogue in this case to pursuing a more objective

understanding of the same phenomena by abandoning the initial subjective viewpoint toward them in

favour of another that is more objective but concerns the same thing? Certainly it appears unlikely that we

will get closer to the real nature of human experience by leaving behind the particularity of our human

point of view and striving for a description in terms accessible to beings that could not imagine what it

was like to be us. If the subjective character of experience is fully comprehensible only from one point of

view, then any shift to greater objectivity—that is, less attachment to a specific viewpoint—does not take

us nearer to the real nature of the phenomenon: it takes us farther away from it.

In a sense, the seeds of this objection to the reducibility of experience are already detectable in successful

cases of reduction; for in discovering sound to be, in reality, a wave phenomenon in air or other media, we

leave behind one viewpoint to take up another, and the auditory, human or animal viewpoint that we leave

behind remains unreduced. Members of radically different species may both understand the same physical

events in objective terms, and this does not require that they understand the phenomenal forms in which

those events appear to the senses of members of the other species. Thus it is a condition of their referring

to a common reality that their more particular viewpoints are not part of the common reality that they both

apprehend. The reduction can succeed only if the species-specific viewpoint is omitted from what is to be

reduced.

But while we are right to leave this point of view aside in seeking a fuller understanding of the external

world, we cannot ignore it permanently, since it is the essence of the internal world, and not merely a

point of view on it. Most of the neobehaviorism of recent philosophical psychology results from the effort

to substitute an objective concept of mind for the real thing, in order to have nothing left over which

cannot be reduced. If we acknowledge that a physical theory of mind must account for the subjective

character of experience, we must admit that no presently available conception gives us a clue how this

could be done. The problem is unique. If mental processes are indeed physical processes, then there is

something it is like, intrinsically, 11 to undergo certain physical processes. What it is for such a thing to be

the case remains a mystery.

What moral should be drawn from these reflections, and what should be done next? It would be a mistake

to conclude that physicalism must be false. Nothing is proved by the inadequacy of physicalist hypotheses

that assume a faulty objective analysis of mind. It would be truer to say that physicalism is a position we

cannot understand because we do not at present have any conception of how it might be true. Perhaps it

will be thought unreasonable to require such a conception as a condition of understanding. After all, it

might be said, the meaning of physicalism is clear enough: mental states are states of the body; mental

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