"Consciousness fits uneasily into our conception of the natural world. On the most common conception of nature, the natural world is the physical world. But on the most common conception of consciousness, it is not easy to see how it could be part of the physical world. So it seems that to find a place for consciousness within the natural order, we must either revise our conception of consciousness, or revise our conception of nature." (p.247)

Easy Problems vs. The Hard ProblemThe easy problems of consciousness involve explaining all the complex computational and cognitive processes that are associated with consciousness--things like: rational control of behavior, monitoring internal states, verbal reports of experience, etc.

The hard problem of consciousness comes to this: explaining why all that cognitive processing feels like something. It sure doesn't seem like conscious experiences can be directly read off from any physical/functional description. But so then what is it about our world that makes all that physical activity come along with conscious experience? What is it about our world that makes us non-zombies?

An Epistemic GapThere are three closely-related arguments which, in their own way, reveal an epistemic gap between the physical and the phenomenal:

• The Explanatory Argument: Physicalist accounts can at most explain the structural/functional features of the world, but that's a poor match for the thing we're trying to explain, phenomenal consciousness...
• The Conceivability Argument: There's no internal contradiction in the idea of a zombie world, and so our world and the zombie world must be distinct metaphysical possibilities...
• The Knowledge Argument: The truths about consciousness are not deducible from mere physical truths...

The general form of these arguments:

1. There is an epistemic gap between physical and phenomenal truths.
2. If there is an epistemic gap between physical and phenomenal truths, then there is an ontological gap, and materialism is false.
3. Conclusion: Materialism is false.

The basic upshot: there's a mismatch in the concepts we use talk/think about consciousness and the concepts we use in physical/functional/structural descriptions. It can then seem impossible to explain the phenomenal facts of the word in terms of the dynamical and structural aspect of the 'underlying' physical world. While we can assert some brute connection between the two kinds of facts/descriptions, that won't tell us why that brute connection exists.

Materialist AccountsType-A Materialism denies the existence of epistemic gap between the phenomenal and the physical. This can take two forms: conceptual functionalism ("our phenomenal concepts are ultimately just functional concepts") or eliminitivism/illusionism ("our phenomenal concepts point to something over and above the basic cognitive functions of the brain, but nothing like that exists"). Both versions simply deny the existence of the hard problem, thinking that once we've solved the easy problems, there's nothing left over to explain.

• Basic Problem: Seems to deny the coherence of scenarios that are clearly coherent (zombies, inverts). They need to provide some strong positive reason to deny the coherence of these seemingly-coherent scenarios, but that seems hard to come by.

Type-B Materialism (empirical functionalism) accepts the existence of an epistemic gap, but denies that it entails an ontological gap. That is, while there's a difference in the way we conceive of cognitive functioning vs. phenomenal experience, the two can nevertheless be revealed by science to be one and the same thing.

• Basic Problem 1: Other successful empirical reductions worked differently. They worked by explaining some high-level functional features in terms of other lower-level functional features. And so the analogy to other empirically-supported identities fails.
• Basic Problem 2: The move to a brute phenomenal-physical identities is ad hoc and unmotivated. It's the move you'd make to save materialism--but why save materialism? There should be some positive motivation.

Type-C Materialism accepts the appearance of an epistemic gap, but holds out hope for that gap to be closed by some future theoretical progress.

• Basic problem: "epistemic implication from A to B requires some sort of conceptual hook by virtue of which the condition described in A can satisfy the conceptual requirements for the truth of B. ... Once we accept that the concept of consciousness is not itself a functional concept, and that physical descriptions of the world are structural-dynamic descriptions, there is simply no conceptual room for it to be implied by a physical description" (260)

Non-Materialist AccountsType-D Dualism (interactionism) holds that phenomenal aspects of the world causally interact with the physical aspects of the world, despite having it's own separate kind of existence.

• Basic Problem: The causal closure of the physical. That is, physics tells us that physical effects have physical causes. There's no room for some external influence on physical systems. All physical activity is explicable in the
• Chalmers Reply: Some interpretations of quantum mechanics make room for observation playing a key role in triggering the collapse of the wave function. More work needs to be done, but it's not incoherent for physical process to be tangled up with the experiences of observers.

Type-E Dualism (epiphenomenalism) "holds that phenomenal properties are ontologically distinct from physical properties, and that the phenomenal has no effect on the physical" (263)

• Basic Problem: Epiphenomenalism makes consciousness into this dangling bit of reality. Why is it there? Note that you and your zombie twin, on this view, behave the exact same way, for all the same physical reasons, it's just that you happen to enjoy this extra phenomenal thing for some unexplained reasons. Why not just think you're a zombie? That we're not zombies is this unexplained 'miracle'.
• Chalmers Reply: You and your zombie twin actually have different beliefs, because your beliefs about consciousness are tied directly to your first person experience, whereas your zombie twins are not.

Type-F Monism (Russellian Monism) holds that the basic building blocks of the universe have some mental aspect to them, which is responsible for consciousness in our universe. While physics captures the the structure and dynamics of these basic building blocks, it says nothing directly about their intrinsic nature.

• Basic Problem: Our only conception of the phenomenal comes from our own first-person experience, and we have no clue how unified points of view emerge from all these little bits of mentality sprinkled throughout the universe. (The combination problem)