Opinionator | The Stone: Mary and the Zombies: Can Science Explain Consciousness?

The Stone is a forum for contemporary philosophers on issues both timely and timeless.

We trust science because its claims are based on experience. But experience itself is a subjective reality that seems to elude the objectivity of scientific understanding. We know that our experiences—of seeing red, feeling pain, falling in love and so forth—depend on physical systems like the brain that science can, in principle, exhaustively explain. But it's hard to make sense of the idea that experiences themselves could be physical. No doubt experiences correlate with objective physical facts, but, as subjective phenomena, how could they be such facts? When I feel intense pain, scientists may be able to observe brain-events that cause my pain, but they cannot observe the very pain that I feel. What science can observe is public and objective; what I feel is private and subjective.

Two famous thought experiments suggest that experience cannot be physical

Perhaps, though, it's hard to understand how experiences could be physical only because we lack the relevant concepts. After all, before scientists developed the concepts of biochemistry, many found it impossible to imagine how organisms could be entirely physical. Now the idea is far more plausible. What's to say that future scientific developments won't allow us to understand how experiences can be entirely physical?

Nevertheless, some of the most interesting recent philosophical discussions have centered around two thought experiments suggesting that experiences cannot be physical. These thought-experiments have by no means settled the issue, but they have convinced quite a few philosophers and have posed a serious challenge to the claim that experience is entirely physical.

First, consider Mary, a leading neuroscientist who specializes in color perception. Mary lives at a time in the future when the neuroscience of color is essentially complete, and so she knows all the physical facts about colors and their perception. Mary, however, has been totally color-blind from birth. (Here I deviate from the story's standard form, in which—for obscure reasons—she's been living in an entirely black-and-white environment.)

Fortunately, due to research Mary herself has done, there is an operation that gives her normal vision. When the bandages are removed, Mary looks around the room and sees a bouquet of red roses sent by her husband. At that moment, Mary for the first time experiences the color red and now knows what red looks like. Her experience, it seems clear, has taught her a fact about color that she did not know before. But before this she knew all the physical facts about color. Therefore, there is a fact about color that is not physical. Physical science cannot express all the facts about color.

Second, consider a zombie. Not the brain-eating undead of movies, but a philosophical zombie, defined as physically identical to you or me but utterly lacking in internal subjective experience. Imagine, for example, that in some alternative universe you have a twin, not just genetically identical but identical in every physical detail—made of all the same sorts of elementary particles arranged in exactly the same way. Isn't it logically possible that this twin has no experiences?

It may, of course, be true that, in our world, the laws of nature require that certain objective physical structures be correlated with corresponding subjective experiences. But laws of nature are not logically necessary (if they were, we could discover them as we do laws of logic or mathematics, by pure thought, independent of empirical facts). So in an alternative universe, there could (logically) be a being physically identical to me but with no experiences: my zombie-twin.

But if a zombie-twin is logically possible, it follows that my experiences involve something beyond my physical makeup. For my zombie-twin shares my entire physical makeup, but does not share my experiences. This, however, means that physical science cannot express all the facts about my experiences.

It's worth noting that that philosophers who find these thought-experiments convincing do not conclude that there is no sense in which an experience is physical. Seeing red, for example, involves photons striking the retina, followed by a whole string of physical events that process the retinal information before we actually have a subjective sense of color. There's a purely physical sense in which this is "seeing." This is why we can say that a surveillance camera "sees" someone entering a room. But the "seeing" camera has no subjective experience; it has no phenomenal awareness of what it's like to see something. That happens only when we look at what the camera recorded. The claim that experience is not physical applies only to this sense of experience. But, of course, it is experience in this sense that makes up the rich inner experience that matters so much to us.

Also, few philosophers think these thought-experiments show that there are souls or some other sort of supernatural entities. Frank Jackson, who first proposed the Mary scenario, and David Chalmers, who gave the most influential formulation of the zombie example, remain philosophical naturalists. They maintain that there is no world beyond the natural one in which we live. Their claim is rather that this world contains a natural reality (consciousness) that escapes the scope of physical explanation. Chalmers, in particular, supports a "naturalistic dualism" that proposes to supplement physical science by postulating entities with irreducibly subjective (phenomenal) properties that would allow us to give a natural explanation of consciousness. Not surprisingly, however, some philosophers have seen Jackson's and Chalmers's arguments as supporting a traditional dualism of a natural body and a supernatural soul.

I myself have come to no firm conclusions about the questions raised by these thought experiments, and I would be very interested in the ideas and arguments of Stone readers. We previously had what I found to be a very fruitful discussion of disagreement among epistemic peers, which convinced me that there could be value in "crowd-sourcing" current philosophical topics. Of course, professional philosophers have technical resources that non-philosophers lack. But a profession brings with it regulating assumptions that themselves should be open to philosophical questioning, and intelligent people outside the profession can be a source of fruitful new ideas. So I encourage readers to share their thoughts on the question: do these thought experiments show that there are facts about consciousness that go beyond what physical science can know—that there is a non-physical aspect to consciousness? I'll discuss the responses in a follow-up column next week.

Gary Gutting is a professor of philosophy at the University of Notre Dame, and an editor of Notre Dame Philosophical Reviews. He is the author of, most recently, "Thinking the Impossible: French Philosophy since 1960," and writes regularly for The Stone.