His mistake is to go further, and conclude that physical goings-on can’t possibly be conscious goings-on. Many make the same mistake today — the Very Large Mistake (as Winnie-the-Pooh might put it) of thinking that we know enough about the nature of physical stuff to know that conscious experience can’t be physical. We don’t. We don’t know the intrinsic nature of physical stuff, except — Russell again — insofar as we know it simply through having a conscious experience.

We find this idea extremely difficult because we’re so very deeply committed to the belief that we know more about the physical than we do, and (in particular) know enough to know that consciousness can’t be physical. We don’t see that the hard problem is not what consciousness is, it’s what matter is — what the physical is.

We may think that physics is sorting this out, and it’s true that physics is magnificent. It tells us a great many facts about the mathematically describable structure of physical reality, facts that it expresses with numbers and equations (e = mc2, the inverse-square law of gravitational attraction, the periodic table and so on) and that we can use to build amazing devices. True, but it doesn’t tell us anything at all about the intrinsic nature of the stuff that fleshes out this structure. Physics is silent — perfectly and forever silent — on this question.

This point was a commonplace one 100 years ago, but it has gotten lost in the recent discussion of consciousness. Stephen Hawking makes it dramatically in his book “A Brief History of Time.” Physics, he says, is “just a set of rules and equations.” The question is what “breathes fire into the equations and makes a universe for them to describe?” What is the fundamental stuff of physical reality, the stuff that is structured in the way physics reveals? The answer, again, is that we don’t know — except insofar as this stuff takes the form of conscious experience.

We can say that it is energy that breathes fire into the equations, using the word “energy” as Heisenberg does when he says, for example, that “all particles are made of the same substance: energy,” but the fundamental question arises again — “What is the intrinsic nature of this energy, this energy-stuff?” And the answer, again, is that we don’t know, and that physics can’t tell us; that’s just not its business. This point about the limits on what physics can tell us is rock solid, and it arises before we begin to consider any of the deep problems of understanding that arise within physics — problems with “dark matter” or “dark energy,” for example — or with reconciling quantum mechanics and general relativity theory.

Those who make the Very Large Mistake (of thinking they know enough about the nature of the physical to know that consciousness can’t be physical) tend to split into two groups. Members of the first group remain unshaken in their belief that consciousness exists, and conclude that there must be some sort of nonphysical stuff: They tend to become “dualists.” Members of the second group, passionately committed to the idea that everything is physical, make the most extraordinary move that has ever been made in the history of human thought. They deny the existence of consciousness: They become “eliminativists.”

This amazing phenomenon (the denial of the existence of consciousness) is a subject for another time. The present point — it’s worth repeating many times — is that no one has to react in either of these ways. All they have to do is grasp the fundamental respect in which we don’t know the intrinsic nature of physical stuff in spite of all that physics tells us. In particular, we don’t know anything about the physical that gives us good reason to think that consciousness can’t be wholly physical. It’s worth adding that one can fully accept this even if one is unwilling to agree with Russell that in having conscious experience we thereby know something about the intrinsic nature of physical reality.

So the hard problem is the problem of matter (physical stuff in general). If physics made any claim that couldn’t be squared with the fact that our conscious experience is brain activity, then I believe that claim would be false. But physics doesn’t do any such thing. It’s not the physics picture of matter that’s the problem; it’s the ordinary everyday picture of matter. It’s ironic that the people who are most likely to doubt or deny the existence of consciousness (on the ground that everything is physical, and that consciousness can’t possibly be physical) are also those who are most insistent on the primacy of science, because it is precisely science that makes the key point shine most brightly: the point that there is a fundamental respect in which ultimate intrinsic nature of the stuff of the universe is unknown to us — except insofar as it is consciousness.