This lesson is about consciousness and not about free will. Whether your choices are determined and whether your experiences are physical are separate questions with separate literatures, and the free-will course handles the first. Running them together is the standard undergraduate error: a determinist can think experience is irreducible, and a libertarian can be a strict physicalist.
The reason consciousness appears in this course is structural. Idealism in lesson seven needs minds to be fundamental. The simulation argument in lesson three needs minds to be implementable in silicon. One argument for God in lesson ten treats consciousness as something physics cannot deliver. All three are hostage to what is settled here, which is why the question cannot be skipped.
The easy problems and the hard one
David Chalmers drew the distinction in 1995 and the vocabulary stuck. The easy problems are explaining the functions: how the brain discriminates stimuli, integrates information, reports its own states, controls behaviour, attends, remembers. They are called easy not because they are simple, since they are the work of neuroscience for the next century, but because we know what a solution would look like. You specify a mechanism that performs the function, and when the mechanism is found the problem is over.
The hard problem is why any of that is accompanied by experience. There is something it is like to see red, and it is not obvious that any account of wavelength discrimination explains why the process is accompanied by that rather than by nothing at all. The worry is that you could give a complete functional story and the question would remain open.
Thomas Nagel set the problem up in "What Is It Like to Be a Bat?" (1974). Bats navigate by echolocation, we know a great deal about how, and none of it tells us what the bat's experience is like from the inside. Nagel's point was not that bat consciousness is mysterious in a way ours is not, but that objective description systematically leaves out the subjective character of experience, and that this is a general feature rather than a gap in current knowledge.
Example. Someone says the hard problem is a failure of imagination, and that vitalism looked just as hard until biochemistry explained life. Is the parallel good?
It is the best objection in the area and it is not decisive. The parallel is real: vitalists insisted that no arrangement of chemistry could produce life, and they were wrong, and the explanation worked by showing that being alive just is a set of functions performed. The disanalogy is what "life" turned out to mean. Every phenomenon vitalists pointed at, metabolism, reproduction, growth, was functional, so explaining the functions explained the thing. The hard problem is defined by the claim that experience is not exhausted by function, so the same move does not obviously apply. Whether that claim is true is the dispute; the vitalism parallel assumes it is false rather than showing it. Notice both sides can state this cleanly, which is a sign of a real disagreement rather than a confusion.
Now you. Why does the hard problem, if genuine, cause trouble for the simulation argument of lesson three?
Answer
Because the simulation argument needs substrate independence, and substrate independence is roughly the claim that running the right computation is sufficient for experience. If experience is exhausted by function, that is plausible, since a simulation reproduces the functions. If the hard problem is genuine, then reproducing every function leaves the question of experience open, and a simulated population might be functionally identical to us with nobody inside. The counting argument then has nothing to count. This is why the two lessons belong to one course: an argument in cosmology turns out to depend on a dispute in philosophy of mind, and someone who accepts the hard problem and also accepts the simulation trilemma is holding two positions that sit badly together.
Three arguments for a gap
The claim that physics cannot deliver consciousness rests on three arguments, all from the 1970s and 1980s, all still argued about.
The knowledge argument, from Frank Jackson in 1982. Mary is a brilliant scientist who has lived her whole life in a black and white room and has learned every physical fact about colour vision: wavelengths, cone responses, the complete neuroscience. She is released and sees a ripe tomato. Does she learn something? The intuition is that she does, that she learns what red looks like. If she knew all the physical facts and still learned something, there are non-physical facts.
The conceivability argument, from Chalmers. A philosophical zombie is a being physically identical to you, atom for atom, which has no experience at all: the lights are off inside. If such a being is coherently conceivable, then experience does not follow logically from the physical facts, and physicalism, which says the physical facts fix everything, is false.
The explanatory gap, from Joseph Levine in 1983. This is the weakest and most robust of the three. Even granting physicalism, there is a gap in our understanding: we cannot see why the physical story entails the experiential one, in the way we can see why the molecular story entails that water is a liquid at room temperature. Levine's claim is about explanation rather than metaphysics, which is what makes it hard to dismiss.
The standard replies are worth knowing. Against Mary: she gains an ability or a new way of representing an old fact rather than a new fact. Against zombies: conceivability is a poor guide to possibility, and the apparent coherence of a zombie may reflect how we think about experience rather than how experience is. Against the gap: it may be a permanent limitation of creatures like us rather than evidence about the world, a position called mysterianism and associated with Colin McGinn.
Example. Mary leaves her room, sees the tomato, and says "so that is what red looks like". A physicalist replies that she has gained an ability rather than a fact. What would settle which is right?
Almost nothing observable, and that is the honest answer. Both sides predict exactly the same behaviour from Mary: the surprise, the sentence, the new capacity to recognise red on sight. The dispute is over how to describe what happened, and by the standard of lesson five two hypotheses that predict identical observations are not separated by those observations. That does not make the question empty, since the descriptions have different consequences elsewhere, notably for whether a complete physics would be a complete account of the world. But it explains why forty years of argument has not converged, and it tells you what to be suspicious of: anyone claiming that a neuroscience result has settled the knowledge argument has almost certainly mistaken a claim about mechanism for a claim about completeness.
Now you. Chalmers says zombies are conceivable, so physicalism is false. Where exactly does an opponent break the argument?
Answer
At the step from conceivability to possibility, which is the only joint in the argument. Nobody disputes that zombies feel conceivable, and the physicalist need not dispute that if they were genuinely possible physicalism would be false. The reply is that conceivability is an unreliable guide: it was once conceivable that water was not H2O, in the sense that no contradiction was apparent, and yet it is not possible, because water is H2O necessarily even though we learned it empirically. Kripke established that shape of case, and the physicalist claims consciousness is another instance, where an a posteriori necessity looks contingent from the inside. Chalmers has an elaborate reply involving two dimensions of meaning, which is why this is a genuine standoff rather than a knockdown, but a reader should be able to name the joint.
The positions
Physicalism holds that experience is physical, either identical to brain states or realised by them. Its burden is the three arguments above.
Dualism holds that experience is a further ingredient. Its burden is causal: if experience is non-physical yet makes you say "I am conscious", it must affect the physical world, and physics as we have it has no room for an extra cause. Property dualism, which keeps one substance and two kinds of property, is the usual modern form and inherits a weaker version of the same problem.
Panpsychism holds that experience is a fundamental feature of matter, present in simple form everywhere and combining into complex forms in brains. It is not the view that electrons have opinions. It takes seriously Russell's point from lesson seven, that physics describes structure and leaves intrinsic nature open, and proposes that the intrinsic nature is experiential. Its burden is the combination problem: nobody can say how simple experiences add up to a single unified one.
Illusionism, defended by Keith Frankish and in a different form by Daniel Dennett, holds that the hard problem is an artefact of introspection, which misrepresents our own states as having properties they lack. There is no experience of the sort the hard problem is about, and what needs explaining is why we are so convinced there is. Its burden is that it appears to deny the one thing nobody can doubt, though its defenders reply that it denies only a theory about that thing.
Example. A friend argues: "Consciousness cannot be physical, and the only remaining explanation is God, so the hard problem is evidence for theism." Evaluate the inference.
Two failures. First, "the only remaining explanation" ignores three live alternatives just listed, since panpsychism, property dualism and illusionism are all non-theistic and all compatible with the hard problem being genuine. An argument from elimination has to eliminate. Second, and by the machinery of lesson five, theism must make consciousness more likely than its rivals do for this to count as evidence, and it is not clear that it does: a deity might create zombies, or minds of some other design, and no one has specified what a deity makes probable here. What survives is much weaker and still worth saying: if consciousness is not physical, a purely physicalist picture of reality is incomplete, which is a real result and not a theistic one.
Now you. Which position in this lesson is most congenial to idealism from lesson seven, and does it rescue it?
Answer
Panpsychism, and it does not rescue idealism so much as replace it with something better behaved. Both hold that experience is fundamental rather than derived, which is the shared commitment, and that is why panpsychism is sometimes described as idealism with the physics kept. The difference is decisive: panpsychism keeps mind-independent things, namely the bearers of the fundamental experiential properties, so the table does not depend on being perceived and the objection that sank Berkeley never arises. Idealism proper says there are no such bearers. So a reader attracted to idealism by the thought that mind is fundamental can have that thought without the costs, which is a good illustration of why mapping positions carefully is worth the effort: the attractive part of a view often detaches from the part that fails.