Hard Problem: The Explanatory Gap

A complete scientific account of tasting coffee seems possible in principle, and it still seems to leave something out. Chemistry can list the compounds that reach your tongue and nose. Physiology can trace how receptors respond to them and how signals travel to the brain. Neuroscience can say which regions respond, how you tell this coffee from another, and why you can report that it's bitter. Suppose every one of those stories were complete. Something would still seem unexplained: that the coffee tastes like anything at all.

That residue is the target of one of the most discussed problems in philosophy of mind. David Chalmers called it the hard problem of consciousness (Chalmers 1995). It asks why and how physical processes are accompanied by experience. The question isn't which brain process goes with which taste. It's why any process goes with any taste, rather than running "in the dark," with no experience at all.

The problem matters for research because it bears on what current methods can deliver. Consciousness science has made real progress in finding the neural processes that correlate with conscious states. It has tools for measuring discrimination, report, attention, and their neural bases. Whether those tools can, even in principle, explain why the processes are experienced is exactly what's in dispute. Some researchers think the hard problem will dissolve as the science matures. Others think it marks a limit that no amount of functional detail will cross. Either answer shapes what a research program promises and what counts as success. A lab that aims to find the neural basis of an experience is making a different promise from one that aims to explain why the experience exists.

The hard problem is also a frequent reference point outside philosophy. Surveys of the field treat it as one of the organizing questions of consciousness studies (Van Gulick 2014). Neuroscientists cite it when they describe what their theories do and don't claim. Public debates about animal minds, brain injury, and artificial intelligence often turn on it without naming it.

A precise statement of the problem protects you from confusion in all of these debates. Without one, it's easy to slide between different questions: whether the brain is complicated, whether a function can be modeled, and whether experience can be explained. Those questions have different answers. A researcher who can state which one a claim addresses can see what the claim shows, what it assumes, and what it leaves open. That skill starts with the arguments that first made the problem sharp. This content reflects the field as of September 2026.

References

• Chalmers, David J. 1995. "Facing Up to the Problem of Consciousness." Journal of Consciousness Studies 2 (3): 200–219.
  • Free: Full text on consc.net
• Van Gulick, Robert. 2014. "Consciousness." In The Stanford Encyclopedia of Philosophy, edited by Edward N. Zalta. https\://plato.stanford.edu/entries/consciousness/.

Introduction

Most discussions of consciousness still start from a phrase Thomas Nagel made famous in 1974: there is something it is like to be a conscious creature. The phrase is simple. The argument Nagel built on it is not, and it's often misreported as a proof that physicalism is false.

This reading explains Nagel's account of the subjective character of experience, his bat example, and what he concludes about physical explanation. Citations give page numbers from the original journal article.

Something It Is Like

Nagel proposes a mark of conscious experience. An organism has conscious mental states, he says, if and only if there is something it is like to be that organism, something it is like for the organism (Nagel 1974, 436). Philosophers now call experience in this sense phenomenal consciousness: experience in the sense that there is something it is like to have it.

Nagel calls this feature the subjective character of experience: in Nagel, the feature of a conscious state that there is something it is like for its subject to be in it, which is tied to a single type of point of view. The last clause is the heart of his argument. A point of view, in Nagel, is a type of perspective, shared by beings of a kind, from which certain facts about experience are accessible. It isn't private to one individual. You and another person can know what it's like for each other to see red. But the facts about what an experience is like are accessible only from a point of view of the right type.

The Bat

To make the point vivid, Nagel chooses a creature that is clearly conscious but very unlike us. Bats find their way largely by echolocation. They emit high-pitched sounds and perceive objects by the reflected echoes. This sense is unlike any we have.

Nagel asks you to try to imagine being a bat. You can imagine having webbed arms, hanging upside down, and hearing echoes. But that only tells you what it would be like for you to behave as a bat behaves. It doesn't tell you what it is like for a bat to be a bat (Nagel 1974, 439). Your imagination starts from your own point of view and can't leave it.

The example carries a step in the argument. It shows that there are facts about experience that a being with our point of view can't grasp, even though they are perfectly real facts about bats.

Objectivity and Its Limit

Physical science aims at an objective standpoint: the perspective physical science aims at, which abstracts from any particular point of view so that its descriptions are available to any observer. Its progress has come from moving away from how things appear to us. A physicist can understand lightning as electrical discharge without seeing or hearing it.

For most phenomena, that move loses nothing. Appearances are one thing and the physical process is another, and the process is what explanation targets. Nagel argues that experience is the exception. Its character just is how it appears from a point of view. If you abstract from the point of view, you leave behind the very thing you set out to explain (Nagel 1974, 444–45).

So the shift Nagel describes runs from the success of objective description elsewhere to its apparent failure here. Reduction usually works by moving toward objectivity. With experience, moving toward objectivity moves away from the phenomenon.

What Nagel Does and Doesn't Conclude

Nagel doesn't claim that physicalism is false. His claim is more careful. We don't yet have any conception of how physicalism could be true of experience. He says that physicalism, as things stand, is a position we can't understand, not one we can reject (Nagel 1974, 446). What's missing is a way of thinking that links the subjective and the objective. He ends by suggesting that such a way of thinking might someday be developed.

This makes Nagel's argument an argument about understanding. It sets up a gap without deciding whether the gap lies in our concepts or in reality.

Conclusion

Nagel identifies phenomenal consciousness with there being something it is like for a creature to have its experience. That subjective character is tied to a type of point of view, which the bat example shows we can't adopt at will. Because physical science abstracts from points of view, it seems to leave experience out. Nagel concludes that we don't understand how physicalism could be true, not that it is false.

Key Terms

• Phenomenal consciousness: Experience in the sense that there is something it is like to have it.
• Subjective character of experience: In Nagel, the feature of a conscious state that there is something it is like for its subject to be in it, which is tied to a single type of point of view.
• Point of view: In Nagel, a type of perspective, shared by beings of a kind, from which certain facts about experience are accessible.
• Objective standpoint: The perspective physical science aims at, which abstracts from any particular point of view so that its descriptions are available to any observer.

References

• Nagel, Thomas. 1974. "What Is It Like to Be a Bat?" Philosophical Review 83 (4): 435–50.
  • Free: None; often readable through a library or a free JSTOR personal account, depending on eligibility.
  • Publisher: doi:10.2307/2183914

Introduction

Suppose pain simply is a certain neural state. Would that settle the question of consciousness? Joseph Levine argued in 1983 that it wouldn't, and Frank Jackson argued a year earlier that the question would still have a factual answer physics leaves out.

This reading explains Levine's explanatory gap, Jackson's knowledge argument, and the distinction between an epistemic gap and an ontological one.

Levine: A Gap in Explanation

Levine starts from a standard physicalist claim: pain is the firing of certain nerve fibers (Levine 1983). He doesn't deny that this identity could be true. His point concerns what it would explain.

Compare a successful scientific identity: heat is the motion of molecules. Once you know how molecular motion works, you can see why anything that has it will behave as heat does. It will warm things, expand gases, and so on. The identity explains. Nothing about heat's causal role is left over.

Now take pain. Suppose you know everything about the neural state: its structure, its causes, and its effects. You still can't see why that state should feel painful, or feel like anything. The physical story tells you what the state does. It doesn't make it intelligible that the state feels the way it does.

Levine calls this the explanatory gap: Levine's term for our inability to see why a given physical state should feel the way it does, even if the two are identical. The gap is in our explanation, not necessarily in the world. Levine is explicit that materialism may be true. What's missing is an account that makes the identity intelligible (Levine 1983).

Jackson: A Gap in Knowledge

Jackson's argument aims further (Jackson 1982). He asks you to consider Mary, a brilliant scientist who has spent her life in a black-and-white room. She has learned, from books and black-and-white screens, every physical fact about color vision: the wavelengths, the retina, and the brain processes. Then she leaves the room and sees a ripe tomato for the first time.

Does she learn something new? Jackson says she does. She learns what it is like to see red. If she knew all the physical facts before, and now knows a new fact, then not all facts are physical. So physicalism is false. This is the knowledge argument: Jackson's argument that a scientist who knows all the physical facts about color vision still learns something new on first seeing red, so that not all facts are physical.

Jackson's paper frames the new knowledge in terms of qualia: the qualitative, felt properties of experiences, such as the redness of seeing red or the painfulness of pain. His conclusion is that qualia are not physical.

The argument has drawn many replies (Ó Conaill 2024). One reply says Mary gains new abilities, such as recognizing and imagining red, not new facts. Another says she gains a new way of thinking about an old physical fact. Jackson himself later abandoned the argument and became a physicalist (Ó Conaill 2024).

Epistemic and Ontological Gaps

The two arguments point to a distinction that runs through the whole debate.

An epistemic gap is a limit in what we can know or understand, which leaves open whether anything is missing from reality itself. Levine's explanatory gap is epistemic. It says we can't see why a physical state feels as it does. It's compatible with that state being all there is.

An ontological gap is a difference in what exists, such as experiences being something over and above physical states. Jackson's argument aims at this. It moves from what Mary didn't know to what there is.

The shift from Levine to Jackson is a shift from a claim about explanation to a claim about reality. Whether an epistemic gap supports an ontological one is exactly what the later debate turns on.

Conclusion

Levine argues that even a true identity between pain and a neural state leaves an explanatory gap: we can't see why the state feels as it does. Jackson's knowledge argument uses Mary's case to argue that physical knowledge leaves out a fact, so physicalism is false. The first gap is epistemic, and the second is ontological.

Key Terms

• Explanatory gap: Levine's term for our inability to see why a given physical state should feel the way it does, even if the two are identical.
• Knowledge argument: Jackson's argument that a scientist who knows all the physical facts about color vision still learns something new on first seeing red, so that not all facts are physical.
• Qualia: The qualitative, felt properties of experiences, such as the redness of seeing red or the painfulness of pain.
• Epistemic gap: A limit in what we can know or understand, which leaves open whether anything is missing from reality itself.
• Ontological gap: A difference in what exists, such as experiences being something over and above physical states.

References

• Jackson, Frank. 1982. "Epiphenomenal Qualia." Philosophical Quarterly 32 (127): 127–36.
  • Free: None.
  • Publisher: Oxford Academic
  • Free companion: SEP, "Qualia: The Knowledge Argument" (Ó Conaill)
• Levine, Joseph. 1983. "Materialism and Qualia: The Explanatory Gap." Pacific Philosophical Quarterly 64 (4): 354–61.
  • Free: None.
  • Publisher: Wiley
• Ó Conaill, Donnchadh. 2024. "Qualia: The Knowledge Argument." In The Stanford Encyclopedia of Philosophy, edited by Edward N. Zalta and Uri Nodelman. https\://plato.stanford.edu/entries/qualia-knowledge/.

Introduction

Nagel's and Levine's arguments made a gap visible (Nagel 1974; Levine 1983). David Chalmers gave it a name that stuck, the hard problem, and an argument that the gap is in reality, not only in our understanding.

This reading explains Chalmers's distinction between the easy problems and the hard problem, the related distinction between phenomenal and access consciousness, and the zombie argument, step by step. Citations give the section of the 1995 article or the chapter of the 1996 book.

Easy Problems and the Hard Problem

Chalmers begins by dividing the problems of consciousness into two kinds (Chalmers 1995, sec. 2). The easy problems are, in Chalmers, the problems of explaining cognitive functions, such as discrimination, report, and attention, in terms of mechanisms. How does the brain tell one stimulus from another? How does it integrate information, focus attention, or make its states available for report? These are hard scientific questions. They're "easy" only in one sense: we know what an answer would look like. You explain a function by finding a mechanism that performs it (Chalmers 1995, sec. 3).

The hard problem is Chalmers's name for the problem of explaining why and how physical processes are accompanied by experience at all. Suppose all the functions are explained. A further question remains. Why is the performance of those functions accompanied by experience? Why doesn't it all happen without anything being felt?

The shift here is from questions about what a system does to a question about why doing it is experienced. Answering the first kind, however completely, doesn't answer the second.

Two Senses of "Consciousness"

The distinction is easier to hold if you separate two senses of the word. Ned Block drew the distinction that is now standard (Block 1995).

Phenomenal consciousness is experience in the sense that there is something it is like to have it. Access consciousness, in Block's distinction, is the availability of a state's content for use in reasoning, report, and the control of action. A state can be access conscious because of what it does. Whether it's also phenomenally conscious is a different question.

The easy problems concern access and other functions. The hard problem concerns phenomenal consciousness. Much confusion in the field comes from sliding between the two.

The Zombie Argument

Chalmers then argues that the hard problem shows physicalism to be false (Chalmers 1996, ch. 3–4). His argument uses a thought experiment.

A zombie, in the philosophical sense, is a being physically identical to a conscious human but with no experience at all. Your zombie twin would have your brain, your behavior, and even your reports. It would say it tastes coffee. But there would be nothing it is like to be it.

The argument has three steps.

1. A zombie is conceivable. You can coherently think of a world physically just like ours in which there is no experience. No contradiction follows from the description.
2. So a zombie is possible. What is ideally conceivable is, Chalmers argues, metaphysically possible.
3. So physicalism is false. Physicalism holds that the physical facts fix all the facts. If a world with all our physical facts could lack experience, then the physical facts don't fix the facts about experience.

This is a conceivability argument: an argument that moves from the conceivability of a scenario, such as a zombie world, to its possibility, and from its possibility to the falsity of physicalism.

Notice where the argument crosses the line that Levine drew. Step 1 is a claim about what we can coherently think, an epistemic matter. Step 3 is a claim about what exists. Step 2 is the bridge. Much of the debate over the argument concerns whether that bridge holds.

Conclusion

Chalmers separates the easy problems, which concern functions, from the hard problem, which concerns why functioning is accompanied by experience. Block's distinction between access and phenomenal consciousness tracks the same line. The zombie argument moves from the conceivability of a physical duplicate without experience, through its possibility, to the falsity of physicalism.

Key Terms

• Easy problems: In Chalmers, the problems of explaining cognitive functions, such as discrimination, report, and attention, in terms of mechanisms.
• Hard problem: Chalmers's name for the problem of explaining why and how physical processes are accompanied by experience at all.
• Phenomenal consciousness: Experience in the sense that there is something it is like to have it.
• Access consciousness: In Block's distinction, the availability of a state's content for use in reasoning, report, and the control of action.
• Zombie: In the philosophical sense, a being physically identical to a conscious human but with no experience at all.
• Conceivability argument: An argument that moves from the conceivability of a scenario, such as a zombie world, to its possibility, and from its possibility to the falsity of physicalism.

References

• Block, Ned. 1995. "On a Confusion about a Function of Consciousness." Behavioral and Brain Sciences 18 (2): 227–47.
• Chalmers, David J. 1995. "Facing Up to the Problem of Consciousness." Journal of Consciousness Studies 2 (3): 200–219.
  • Free: Full text on consc.net
• Chalmers, David J. 1996. The Conscious Mind: In Search of a Fundamental Theory. New York: Oxford University Press.
  • Free: Only the introduction, on consc.net.
  • Free companion: SEP, "Zombies" (Kirk)
• Levine, Joseph. 1983. "Materialism and Qualia: The Explanatory Gap." Pacific Philosophical Quarterly 64 (4): 354–61.
  • Free: None.
  • Publisher: Wiley
• Nagel, Thomas. 1974. "What Is It Like to Be a Bat?" Philosophical Review 83 (4): 435–50.
  • Free: None; often readable through a library or a free JSTOR personal account, depending on eligibility.
  • Publisher: doi:10.2307/2183914

Introduction

The zombie argument is valid in form, so anyone who rejects its conclusion must reject a premise. Physicalists have tried each one. Chalmers, for his part, later turned the question around and asked why the problem seems so compelling in the first place.

This reading sets out the main replies to each premise, explains the meta-problem, and compares the four arguments that frame the hard problem. This content reflects the field as of September 2026.

Replies to Each Premise

The replies line up with the argument's three steps (Kirk 2023).

1. Deny that zombies are conceivable. Some physicalists hold that once you really understand the functions, there's nothing left for a zombie to lack. The apparent conceivability of zombies, on this view, comes from not imagining the full physical story. A zombie that says it tastes coffee, and processes coffee exactly as you do, has everything tasting consists in.
2. Deny that conceivability implies possibility. Other physicalists grant that zombies are conceivable but deny that they're possible. Water is H₂O, though you can conceive of water not being H₂O. Some truths are necessary even though reasoning alone can't reveal them. Experience could be identical to a physical process in the same way.
3. Deny that the conclusion follows. Some philosophers accept that a zombie world is possible but deny that this refutes every form of physicalism. If physics describes only the structure of matter, not its intrinsic nature, a zombie world might differ from ours in what physics leaves out.

None of these replies is agreed to succeed. Each shifts the burden to a different place: to our concepts, to the link between conceivability and possibility, or to what "physical" means.

The Meta-Problem

Twenty-three years after naming the hard problem, Chalmers posed a second one. The meta-problem is Chalmers's name for the problem of explaining why we judge that there is a hard problem of consciousness (Chalmers 2018, sec. 1).

The meta-problem is about our reports and judgments. People say that experience seems nonphysical, that explaining functions leaves something out, and that a zombie is conceivable. Chalmers calls these dispositions "problem intuitions." Because they show up in behavior and judgment, they are functions. So the meta-problem is, in principle, an easy problem. It can be approached with the ordinary tools of cognitive science.

The shift is from explaining experience to explaining what we say about experience. That shift matters for two reasons (Chalmers 2018, sec. 3). First, it gives researchers with opposed views a shared task. Second, whatever the answer is, it constrains the hard problem. If the mechanisms that produce our problem intuitions would operate whether or not experience is real, that is a point for those who think the problem rests on an illusion. If they can't be explained without experience, that is a point for its reality.

Four Arguments Compared

The table compares the four arguments that set up the hard problem (Nagel 1974; Levine 1983; Jackson 1982; Chalmers 1996).

Core Terms at a Glance

Conclusion

Replies to the zombie argument deny that zombies are conceivable, deny that conceivability implies possibility, or deny that possibility refutes every physicalism. The meta-problem asks why we judge there to be a hard problem. It is itself an easy problem, and its answer bears on the hard one.

Key Terms

• Meta-problem: Chalmers's name for the problem of explaining why we judge that there is a hard problem of consciousness.
• Phenomenal consciousness: Experience in the sense that there is something it is like to have it.
• Access consciousness: In Block's distinction, the availability of a state's content for use in reasoning, report, and the control of action.
• Explanatory gap: Levine's term for our inability to see why a given physical state should feel the way it does, even if the two are identical.
• Easy problems: In Chalmers, the problems of explaining cognitive functions, such as discrimination, report, and attention, in terms of mechanisms.
• Hard problem: Chalmers's name for the problem of explaining why and how physical processes are accompanied by experience at all.
• Conceivability argument: An argument that moves from the conceivability of a scenario, such as a zombie world, to its possibility, and from its possibility to the falsity of physicalism.

References

• Chalmers, David J. 1996. The Conscious Mind: In Search of a Fundamental Theory. New York: Oxford University Press.
  • Free: Only the introduction, on consc.net.
  • Free companion: SEP, "Zombies" (Kirk)
• Chalmers, David J. 2018. "The Meta-Problem of Consciousness." Journal of Consciousness Studies 25 (9–10): 6–61.
  • Free: PDF on consc.net
• Jackson, Frank. 1982. "Epiphenomenal Qualia." Philosophical Quarterly 32 (127): 127–36.
  • Free: None.
  • Publisher: Oxford Academic
  • Free companion: SEP, "Qualia: The Knowledge Argument" (Ó Conaill)
• Kirk, Robert. 2023. "Zombies." In The Stanford Encyclopedia of Philosophy, edited by Edward N. Zalta and Uri Nodelman. https\://plato.stanford.edu/entries/zombies/.
• Levine, Joseph. 1983. "Materialism and Qualia: The Explanatory Gap." Pacific Philosophical Quarterly 64 (4): 354–61.
  • Free: None.
  • Publisher: Wiley
• Nagel, Thomas. 1974. "What Is It Like to Be a Bat?" Philosophical Review 83 (4): 435–50.
  • Free: None; often readable through a library or a free JSTOR personal account, depending on eligibility.
  • Publisher: doi:10.2307/2183914

Introduction

"Facing Up to the Problem of Consciousness" is one of the most cited papers in philosophy of mind, and its key move takes only a few pages. Chalmers separates what can be explained by finding mechanisms from what, he argues, can't. This reading works through that move in four steps, pauses on a phrase that signals the hard problem, flags a common misreading, and ends with a model paraphrase of the distinction.

Locating the Passage

The passage is sections 2 and 3 of the article, "The easy problems and the hard problem" and "Functional explanation" (Chalmers 1995, secs. 2–3). The full text is free on consc.net, so you can read along. Short key phrases are quoted below. Everything else is paraphrased.

Before you start, note the setting. Chalmers opens the paper by saying that the word "consciousness" is used for many different phenomena. His first task is to sort them.

Walking Through the Argument

Step 1: The list of easy problems

Chalmers begins with a list (Chalmers 1995, sec. 2). The easy problems include explaining:

1. the ability to discriminate, categorize, and react to stimuli;
2. the integration of information by a cognitive system;
3. the reportability of mental states;
4. a system's ability to access its own internal states;
5. the focus of attention;
6. the deliberate control of behavior;
7. the difference between wakefulness and sleep.

Notice what the items share. Each names something a system does. Each could be observed from outside, at least in principle, in behavior or in brain activity. And each is a phenomenon that people ordinarily call "consciousness" in some sense. Chalmers's point is that these senses are real, but they aren't the one that makes consciousness puzzling.

Step 2: Why they are "easy": they concern functions

The second move explains what makes the list hang together. Every item is a function, a matter of what role a state or process plays. And functions have a standard form of explanation. To explain a function, Chalmers writes, we need only "specify a mechanism" that can perform it (Chalmers 1995, sec. 3).

Take reportability. To explain it, you'd show how information in the brain comes to drive speech. Once you'd shown that, nothing about reportability would be left over. The explanation of the mechanism is the explanation of the function.

So "easy" names a kind of problem, not a level of difficulty. We know what form a solution would take, even when finding it is very hard.

Step 3: Why experience isn't a function

The third move is the pivot. Chalmers identifies the really hard problem as the problem of "experience" (Chalmers 1995, sec. 2). When you see, there is something it is like to see: the felt quality of color and depth.

Why isn't experience one more function? Because you can specify all the functions without mentioning it. Describe discrimination, integration, report, and attention in full. The description is complete as an account of what the system does. Yet it doesn't yet say that any of this is experienced. Experience isn't defined by a role, so explaining roles doesn't reach it.

The shift here is from "what does the system do?" to "why is its doing accompanied by experience?" The first question has functional answers. The second asks about something functional answers don't mention.

Step 4: The "further question"

The final move draws the consequence (Chalmers 1995, sec. 3). Suppose every easy problem is solved. A further question remains: why is the performance of these functions accompanied by experience? A simple explanation of the functions, Chalmers says, leaves this question open.

This is what makes the hard problem distinctive. For other phenomena, such as life or learning, once the functions are explained there is no further question. Explaining how an organism reproduces, metabolizes, and adapts just is explaining life. Chalmers argues that experience is the exception. The further question survives the full functional story.

Callout: "Why Is It That…"

Watch for a form of question in the text. The hard problem is typically posed as "why" rather than "how": why is it that this processing is accompanied by experience, rather than going on without it? The easy problems are posed as "how": how does the system discriminate, integrate, and report?

When you read any claim to have solved the problem of consciousness, check which form of question it answers. A claim that answers a "how" question about a function has addressed an easy problem, however impressive the result. Only an answer to the "why is it that…" question addresses the hard problem as Chalmers states it.

Key Considerations

Terms.

A common misreading. The most common misreading takes "easy" to mean "simple to solve."

• Chalmers says the opposite. He calls "easy" a relative term and says that getting the details right may take "a century or two of difficult empirical work" (Chalmers 1995, sec. 2).
• "Easy" names a form of problem. We know what a solution would look like: a mechanism that performs the function.
• "Hard" names a different form. The difficulty is that we don't know what a solution would look like, because explaining functions doesn't obviously reach experience.

A second, related misreading takes the hard problem to be that the brain is very complex. Complexity makes the easy problems hard in practice. It doesn't create the hard problem.

Summary

The passage lists the easy problems, identifies them as problems about functions explained by mechanisms, argues that experience isn't a function, and concludes that a further question survives every functional explanation. Here is a model paraphrase of the distinction to check your reading against:

The easy problems of consciousness are problems about functions: how a system discriminates stimuli, integrates information, reports its states, attends, and controls behavior. They are easy only in that we know what form an answer takes: specify a mechanism that performs the function. The hard problem is different. Even when every function is explained, we can still ask why their performance is accompanied by experience, why there is something it is like to be the system. Because experience isn't defined by what it does, explaining what the system does leaves that question open (Chalmers 1995, secs. 2–3).

References

• Chalmers, David J. 1995. "Facing Up to the Problem of Consciousness." Journal of Consciousness Studies 2 (3): 200–219.
  • Free: Full text on consc.net