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Facing Up to the Problem of Consciousness

David J. Chalmers

Philosophy Program

Research School of Social Sciences

Australian National University

1 Introduction

Consciousness poses the most baffling problems in the science of the mind. There is

nothing that we know more intimately than conscious experience, but there is nothing that is

harder to explain. All sorts of mental phenomena have yielded to scientific investigation in

recent years, but consciousness has stubbornly resisted. Many have tried to explain it, but the

explanations always seem to fall short of the target. Some have been led to suppose that the

problem is intractable, and that no good explanation can be given.

To make progress on the problem of consciousness, we have to confront it directly. In

this paper, I first isolate the truly hard part of the problem, separating it from more tractable

parts and giving an account of why it is so difficult to explain. I critique some recent work

that uses reductive methods to address consciousness, and argue that these methods inevitably

fail to come to grips with the hardest part of the problem. Once this failure is recognized, the

door to further progress is opened. In the second half of the paper, I argue that if we move to a

new kind of nonreductive explanation, a naturalistic account of consciousness can be given.

I put forward my own candidate for such an account: a nonreductive theory based on

principles of structural coherence and organizational invariance and a double-aspect view

of information.

2 The Easy Problems and the Hard Problem

There is not just one problem of consciousness. “Consciousness” is an ambiguous term,

referring to many different phenomena. Each of these phenomena needs to be explained, but

some are easier to explain than others. At the start, it is useful to divide the associated

This paper was published in Journal of Consciousness Studies, 2(3):200-19, 1995. Thanks to Francis Crick,

Peggy DesAutels, Matthew Elton, Liane Gabora, Christof Koch, Paul Rhodes, Gregg Rosenberg, and Sharon

Wahl for their comments.

problems of consciousness into “hard” and “easy” problems. The easy problems of

consciousness are those that seem directly susceptible to the standard methods of cognitive

science, whereby a phenomenon is explained in terms of computational or neural

mechanisms. The hard problems are those that seem to resist those methods.

The easy problems of consciousness include those of explaining the following

phenomena:

• the ability to discriminate, categorize, and react to environmental stimuli;

• the integration of information by a cognitive system;

• the reportability of mental states;

• the ability of a system to access its own internal states;

• the focus of attention;

• the deliberate control of behavior;

• the difference between wakefulness and sleep.

All of these phenomena are associated with the notion of consciousness. For example,

one sometimes says that a mental state is conscious when it is verbally reportable, or when it

is internally accessible. Sometimes a system is said to be conscious of some information when

it has the ability to react on the basis of that information, or, more strongly, when it attends to

that information, or when it can integrate that information and exploit it in the sophisticated

control of behavior. We sometimes say that an action is conscious precisely when it is

deliberate. Often, we say that an organism is conscious as another way of saying that it is

awake.

There is no real issue about whether these phenomena can be explained scientifically. All

of them are straightforwardly vulnerable to explanation in terms of computational or neural

mechanisms. To explain access and reportability, for example, we need only specify the

mechanism by which information about internal states is retrieved and made available for

verbal report. To explain the integration of information, we need only exhibit mechanisms by

which information is brought together and exploited by later processes. For an account of

sleep and wakefulness, an appropriate neurophysiological account of the processes

responsible for organisms’ contrasting behavior in those states will suffice. In each case, an

appropriate cognitive or neurophysiological model can clearly do the explanatory work.

If these phenomena were all there was to consciousness, then consciousness would not

be much of a problem. Although we do not yet have anything close to a complete explanation

of these phenomena, we have a clear idea of how we might go about explaining them. This is

why I call these problems the easy problems. Of course, ‘easy’ is a relative term. Getting the

details right will probably take a century or two of difficult empirical work. Still, there is

every reason to believe that the methods of cognitive science and neuroscience will succeed.

The really hard problem of consciousness is the problem of experience . When we think

and perceive, there is a whir of information-processing, but there is also a subjective aspect.

As Nagel (1974) has put it, there is something it is like to be a conscious organism. This

subjective aspect is experience. When we see, for example, we experience visual sensations:

the felt quality of redness, the experience of dark and light, the quality of depth in a visual

field. Other experiences go along with perception in different modalities: the sound of a

clarinet, the smell of mothballs. Then there are bodily sensations, from pains to orgasms;

mental images that are conjured up internally; the felt quality of emotion, and the experience

of a stream of conscious thought. What unites all of these states is that there is something it is

like to be in them. All of them are states of experience.

It is undeniable that some organisms are subjects of experience. But the question of how

it is that these systems are subjects of experience is perplexing. Why is it that when our

cognitive systems engage in visual and auditory information-processing, we have visual or

auditory experience: the quality of deep blue, the sensation of middle C? How can we explain

why there is something it is like to entertain a mental image, or to experience an emotion? It

is widely agreed that experience arises from a physical basis, but we have no good

explanation of why and how it so arises. Why should physical processing give rise to a rich

inner life at all? It seems objectively unreasonable that it should, and yet it does.

If any problem qualifies as the problem of consciousness, it is this one. In this central

sense of “consciousness”, an organism is conscious if there is something it is like to be that

organism, and a mental state is conscious if there is something it is like to be in that state.

Sometimes terms such as “phenomenal consciousness” and “qualia” are also used here, but I

find it more natural to speak of “conscious experience” or simply “experience”. Another

useful way to avoid confusion (used by e.g., Newell 1990; Chalmers 1996) is to reserve the

term “consciousness” for the phenomena of experience, using the less loaded term

“awareness” for the more straightforward phenomena described earlier. If such a convention

were widely adopted, communication would be much easier; as things stand, those who talk

about “consciousness” are frequently talking past each other.

The ambiguity of the term “consciousness” is often exploited by both philosophers and

scientists writing on the subject. It is common to see a paper on consciousness begin with an

invocation of the mystery of consciousness, noting the strange intangibility and ineffability of

subjectivity, and worrying that so far we have no theory of the phenomenon. Here, the topic is

clearly the hard problem—the problem of experience. In the second half of the paper, the tone

becomes more optimistic, and the author’s own theory of consciousness is outlined. Upon

examination, this theory turns out to be a theory of one of the more straightforward

phenomena—of reportability, of introspective access, or whatever. At the close, the author

declares that consciousness has turned out to be tractable after all, but the reader is left feeling

like the victim of a bait-and-switch. The hard problem remains untouched.

3 Functional Explanation

Why are the easy problems easy, and why is the hard problem hard? The easy problems

are easy precisely because they concern the explanation of cognitive abilities and functions .

To explain a cognitive function, we need only specify a mechanism that can perform the

function. The methods of cognitive science are well-suited for this sort of explanation, and so

are well-suited to the easy problems of consciousness. By contrast, the hard problem is hard

precisely because it is not a problem about the performance of functions. The problem persists

even when the performance of all the relevant functions is explained. (Here “function” is not

used in the narrow teleological sense of something that a system is designed to do, but in the

broader sense of any causal role in the production of behavior that a system might perform.)

To explain reportability, for instance, is just to explain how a system could perform the

function of producing reports on internal states. To explain internal access, we need to explain

how a system could be appropriately affected by its internal states and use information about

those states in directing later processes. To explain integration and control, we need to explain

how a system’s central processes can bring information contents together and use them in the

facilitation of various behaviors. These are all problems about the explanation of functions.

How do we explain the performance of a function? By specifying a mechanism that

performs the function. Here, neurophysiological and cognitive modeling are perfect for the

task. If we want a detailed low-level explanation, we can specify the neural mechanism that is

responsible for the function. If we want a more abstract explanation, we can specify a

mechanism in computational terms. Either way, a full and satisfying explanation will result.

Once we have specified the neural or computational mechanism that performs the function of

verbal report, for example, the bulk of our work in explaining reportability is over.

In a way, the point is trivial. It is a conceptual fact about these phenomena that their

explanation only involves the explanation of various functions, as the phenomena are

functionally definable . All it means for reportability to be instantiated in a system is that the

system has the capacity for verbal reports of internal information. All it means for a system to

be awake is for it to be appropriately receptive to information from the environment and for it

to be able to use this information in directing behavior in an appropriate way. To see that this

sort of thing is a conceptual fact, note that someone who says “you have explained the

performance of the verbal report function, but you have not explained reportability” is making

a trivial conceptual mistake about reportability. All it could possibly take to explain

reportability is an explanation of how the relevant function is performed; the same goes for

the other phenomena in question.

Throughout the higher-level sciences, reductive explanation works in just this way. To

explain the gene, for instance, we needed to specify the mechanism that stores and transmits

hereditary information from one generation to the next. It turns out that DNA performs this

function; once we explain how the function is performed, we have explained the gene. To

explain life, we ultimately need to explain how a system can reproduce, adapt to its

environment, metabolize, and so on. All of these are questions about the performance of

functions, and so are well-suited to reductive explanation. The same holds for most problems

in cognitive science. To explain learning, we need to explain the way in which a system’s

behavioral capacities are modified in light of environmental information, and the way in

which new information can be brought to bear in adapting a system’s actions to its

environment. If we show how a neural or computational mechanism does the job, we have

explained learning. We can say the same for other cognitive phenomena, such as perception,

memory, and language. Sometimes the relevant functions need to be characterized quite

subtly, but it is clear that insofar as cognitive science explains these phenomena at all, it does

so by explaining the performance of functions.

When it comes to conscious experience, this sort of explanation fails. What makes the

hard problem hard and almost unique is that it goes beyond problems about the performance

of functions. To see this, note that even when we have explained the performance of all the

cognitive and behavioral functions in the vicinity of experience—perceptual discrimination,

categorization, internal access, verbal report—there may still remain a further unanswered

question: Why is the performance of these functions accompanied by experience? A simple

explanation of the functions leaves this question open.

There is no analogous further question in the explanation of genes, or of life, or of

learning. If someone says “I can see that you have explained how DNA stores and transmits

hereditary information from one generation to the next, but you have not explained how it is a

gene ”, then they are making a conceptual mistake. All it means to be a gene is to be an entity

that performs the relevant storage and transmission function. But if someone says “I can see

that you have explained how information is discriminated, integrated, and reported, but you

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