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

Thanks to the nearly 800 readers who submitted comments on last week’s column, “Mary and the Zombies: Can Science Explain Consciousness?” Together they produced over 100,000 words (a good sized book). The column summarized two widely discussed arguments against understanding consciousness as entirely physical: the Mary Argument and the Zombie Argument.

Many comments found both arguments utterly and obviously inadequate. But the dismissive comments often missed important points, as other comments pointed out. In any case, there were a remarkable number of intelligent and perceptive discussions. I can cite only a very small sample of what was said and apologize to the vast majority whose contributions I don’t mention. The comments I’ve discussed were often similar to others, and many readers not named may notice points that they made.



I begin with the Mary Argument. Privacy Guy thinks philosophers impressed by this argument “need more science classes.” What happens to Mary, he says, “is easily explained by physical phenomenon. Mary cannot experience color without having the neurons in her brain usually associated with the experience of color and having these neurons appropriately stimulated.” This is a good point, but it only shows that Mary’s seeing red depends on physical events in the brain. It doesn’t show that her seeing red is itself physical, and so doesn’t refute the argument.

Rob Johnson offers a more complex analysis of Mary’s situation. He takes the claim that she knows all the physical facts about color to mean that “her brain has encoded all the physical laws governing the perception of color.” As a result, “her brain has encoded descriptions of the physical changes that occur in our brains when we see color,” although “her brain has not gone through these changes itself.” All that happens when she sees red for the first time is that “her brain undergoes the changes she’s learned about.” There is, he says, “nothing unphysical about this.” (Teresa endorses a similar view.)

But is it true that “seeing red” involves nothing more than the brain’s undergoing certain changes? As Dan says in response: “Clearly the brain represents knowledge about red differently from how it actually processes visual signals. The point is, neither one of these processes bear any resemblance to [Mary’s] subjective experience of red . . . What do neurons have to do with redness? Perhaps . . . certain neural activity occurs at the same time as the sensation of redness, but they are fundamentally different things. That is the sense in which redness could be non-physical.”

Many readers see the Mary argument as obviously circular. As lostintranslation puts it: “This is circular reasoning. The statement ‘Mary for the first time experiences the color red and now knows what red looks like’ assumes that Mary’s knowledge of the physical facts did not include the experience of seeing color. . . . If Mary’s pre-sight-experience knowledge of facts was purely linguistic, then, yes, she would not have had the sight-experience knowledge of color, but then, too, she would not have known ‘all the physical facts about colors and their perception.'” But another reader, patalcant, reverses the charge of circularity: “You seem to acknowledge that what you call ‘sight-experience’ knowledge of color is different from what Mary knew before her operation. You simply claim, baldly, that that additional knowledge is a ‘physical’ fact of which she was previously unaware. But there is the circularity, in that very assertion. What is in question is precisely whether that additional fact can correctly be described as physical rather than subjective.”

John suggests that both sides may be right. He offers an account of what happens to Mary in purely physical terms, while allowing that “perhaps this argument is flawed because it builds in the assumption to be tested” (that Mary’s experience is entirely physical). But he also thinks that the Mary argument itself “builds in the opposite assumption.” From this standpoint, the Mary argument may not prove that consciousness is non-physical, but it may show that physicalists and non-physicalists are divided by the different basic assumptions they make about consciousness.

Neither side, however, will be satisfied with this conclusion. Physicalists will maintain that “materialism is well supported by the empirical evidence, and it would require an extremely strong amount of counterevidence to overthrow it” (Dave). Non-physicalists will respond that “a thorough, correct, objective description of a state of the brain will still leave out the experience of what it’s like to have one’s own brain in that state; so the description, it seems, is incomplete” (A Reader; Barefoot Boy makes a similar point).

The physicalist’s picture is that science is gradually closing in on a complete account of consciousness by learning more and more about how consciousness depends on physical structures and processes. The non-physicalist’s picture is that, despite all this information about the connection of consciousness to the physical, science has made no progress at all toward showing that subjective conscious experience is physical. In one sense, each side is betting on how science will develop.

Physicalists are betting that science’s pursuit of physical correlations with experience will somehow eventually converge to a currently mysterious identity of consciousness with these correlations. Non-physicalists back a continuing failure to make the jump from correlation to identity. But there seems to be an element of faith in any prophecies about the future direction of science. We currently have no clear idea how a subjective experience could literally be a purely physical brain event, but who knows what sorts of ideas will develop in the future? Winning the bet depends on whether there will be an entirely new way of thinking about experience that somehow combines the subjective first-person standpoint with the objective third-person standpoint. Apart from just assuming the truth or falsity of physicalism, there is no basis for supporting one rather than the other outcome.

As in the case of Mary, many comments claim that the Zombie argument is circular; that is, it assumes as a premise what it is trying to prove. Bill (Long Beach) has a lucid formulation. The question, he says, is “whether experience is solely part of my physical makeup.” To answer it, “I assume the possible existence of a zombie that is physically identical to me, but has no subjective experience.” This assumption “excludes my experience from my physical substance . . . . For if Zombie = Me – Experience, then Zombie + Experience = Me. And if Zombie = Physical Me, then Experience is non-physical. But the very assumption that such a zombie exists begs the question.”

But exactly where is the circularity? The premise that a zombie is possible is a different claim from the conclusion that my experience is non-physical (for one thing, the premise is about a zombie and the conclusion is not). Bill in fact very deftly shows, through his series of “equations,” just how the premise (along with the further obvious premise that I have experience) leads to the conclusion, rather than simply assuming it. People tend to suspect circularity because, given “Zombies are possible” the inference to “My experience is non-physical” is very quick and easy.

Related More From The Stone Read previous contributions to this series.

This, in fact, is the strength of the Zombie Argument. The logical possibility of a zombie-twin strikes most people as intuitively obvious: what could there be in my overall physical make-up that logically requires that I be conscious? (Even Daniel Dennett, who in the end denies the possibility of zombies, has noted the powerful “tug” of the idea and admits, “I can feel the tug as well as anybody.”) To undermine the argument, critics need to show that the key premise (the possibility of a zombie) is false or at least unjustified. It does no good to complain that the premise entails the conclusion.

Many critics of the Zombie Argument do argue that a zombie-twin is not possible, on the grounds that physically identical beings must have the same experiences. Gemli puts it bluntly: ” The zombie argument is silly, as a physically identical being couldn’t be identical and not have the same experiences.” (A.D. and Herb Sevush make similar points.) But what does “couldn’t” mean here? We know that our experiences are closely tied to our physical make-up, and this may well be because there are laws of nature correlating specific physical states with specific experiences. But laws of nature are not logically necessary (that is, denying them is not a contradiction). There is, for example, no logical contradiction in claiming that planets move around the Sun in circles rather than ellipses. Philosophers, therefore, distinguish the physical (or natural) necessity of laws from logical necessity. The basic premise of the Zombie Argument is that my zombie-twin is logically possible (and therefore that laws of nature connecting physical make-up with experiences are not logically necessary, even if they are physically necessary).

Some readers object that assuming different laws of nature makes my zombie-twin no longer identical with me. JohnB puts the point nicely: “The laws of nature determine the behavior of the particles. If the particles behaved differently, then it wouldn’t really be an exact twin. But if the particles behaved the same, then effectively the laws of nature governing those particles are the same in the alternative universe as in ours.” (MF and others also argue for this point.) This would be true if what changed were laws governing the behavior of the physical make-up my zombie-twin and I share. But the Zombie Argument doesn’t assume such changes. It eliminates only laws connecting physical make-up to experiences. Eliminating these laws eliminates only the experiences correlated with my zombie-twin’s physical make-up. It does nothing to that physical make-up itself.

Greg and zooey make a fascinating, but radical, suggestion. As Greg puts it, “It is not clear that the laws of nature are not logically necessary. It may just be easier to deduce those laws from experience, than from first principle using pure thought.” This would undermine the Zombie Argument, but it would also imply that, in principle, we could establish laws of nature by nothing but pure thought (as in mathematics). This would mean that natural science need not be an empirical enquiry, requiring observation and experiment. It could be a matter of mere armchair reasoning—no better than philosophy.

Reflecting on the discussion overall, my conclusion is that neither the Mary nor the Zombie Argument makes a decisive case against physicalism. But the arguments do not make obvious, stupid mistakes. They are based on premises with a great deal of intuitive appeal (that science can’t tell us what red looks like subjectively and that a zombie is logically possible), and these premises seem to lead quite directly to the conclusion that consciousness is non-physical. That by no means settles the issue. We need to examine much more carefully both the apparently obvious premises and the reasoning based on them. Along these lines, professional philosophers have uncovered a number of subtle and complex problems for both arguments. For anyone interested in pursuing the discussion further, I would recommend the Stanford Encyclopedia of Philosophy articles “Qualia: The Knowledge Argument” (by Martine Nida-Rümelin) and “Zombies” (by Robert Kirk).



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. He was recently interviewed in 3am magazine.

