How to Tell If You’re a Zombie

Estimated read time (minus contemplative pauses): 10 min.

In a recent post, “Some Naive Questions About Many-Worlds Quantum Mechanics,” I express bewilderment at Sean Carroll asking, “How do you know you’re not a zombie?” Where the word “zombie” comes from a thought experiment in which you’re asked to try to conceive of mindless humanoids (i.e., zombies) who behave exactly like conscious humans but are no more conscious than a pile of laundry. (They’re often called philosophical zombies, but I’ll just call them zombies.)

The point of the exercise is to make the experimenter friendlier to some form of dualism—that is, to the idea that minds and brains are not identical in some important sense. For example, if we can conceive of zombies, then maybe the human brain has not only naturally occurring physical properties, but naturally occurring mental ones as well.

But Carroll, who rejects zombies on grounds of inconceivability, takes the experiment in a surprising direction by asking those who don’t reject zombies how they can be sure they aren’t one. I have a hard time conceiving of zombies, but it seems obvious to me that there’s nothing about the zombie experiment that warrants Carroll’s question, which, at the time of the above-referenced post, I had observed him asking in his 2016 book, The Big Picture: On the Origins of Life, Meaning, and the Universe Itself (Chapter 41), and on Sam Harris’s Making Sense podcast (Episode 124: “In Search of Reality” [4/21/18]; also available on YouTube).

Here’s the gist of my response in that post.

I know I’m not a zombie because I experience things. For instance, I experience my own thoughts. Case closed! But if that doesn’t convince you, reflect on the silliness of telling me, “You don’t have thoughts; rather, you have incorrect thoughts whose content is, ‘I have thoughts.'”

I doubt, however, that Carroll means his question literally. I’m sure he does not doubt that he himself experiences things and, again, he rejects the conceivability of zombies. He might, then, mean his question as a coy rhetorical move aimed at urging those who find zombies conceivable to reflect more deeply on that finding and thus correct themselves. (As an added bonus, he might in some instances also be asking us to think more deeply about the mysterious and strangely inaccessible nature of consciousness by noticing how hard it would be to persuade a skeptic that we are conscious. Nice!)

That’s basically where I left things in the “Naive Questions” post. But then I heard Episode 25 of his excellent Mindscape podcast: “David Chalmers on Consciousness, the Hard Problem, and Living in a Simulation” (12/3/18). Chalmers is a hugely influential figure in contemporary discussions of philosophy of mind, both in and outside of academic philosophy. In fact, his is the name most associated with the zombie thought experiment as we know it today.

In that conversation, Carroll raises the question to Chalmers with such sincerity that it strikes me as a genuine attempt to make sense of what Carroll takes to be conceptual difficulties with the zombie experiment. At least, he seems, in good faith, to be respectfully inviting Chalmers (who finds zombies conceivable) to clear things up, and maybe to even change Carroll’s mind. The discussion of zombies starts at about 24:22, but I’m skipping to about 31:00 (listen also at YouTube):

Carroll: How can I be sure that I’m not a zombie if all the things I say and do are exactly what a zombie would say and do?

Chalmers: Well I think there’s a very good argument that I can’t be sure that you’re not a zombie because all I have access to, with respect to you, is your behavior and your functioning and so on. And none of that seems to absolutely differentiate you from a zombie. I think the first-person case is different. Because in the first-person case, I’m conscious. I know that I’m conscious, I know that more directly than I know anything else.

I mean, Descartes said way back in the 1640s, this is the one thing I can be certain of. I can doubt everything about the external world, I can doubt there’s a table here, I can doubt there’s a body. There’s one thing I can’t doubt, that’s that I’m thinking; or I think he could even better have said: that I’m conscious. “I think, therefore I am.” So therefore, I can’t doubt my own existence. So I think it’s natural to take consciousness as our primary epistemic datum.

So, whatever you say about zombies and so on I know that I’m not one of them because I know that I’m conscious.

Carroll: But I think that my worry about exactly that is that so, like you said, my argument certainly would make you wonder whether I am conscious. I think it also makes me wonder whether I am conscious because I think that a zombie me would behave in exactly the same way. This includes writing all the bad poetry I wrote in high school and crying at movies, at WALL-E and so forth, and petting my cats. All of these things the zombie would do in exactly the same way that I do. If you ask that zombie me, “Are you conscious?,” it would say, “Yes, and here’s why.” It would give you reasons. I don’t see how I can be sure that I’m not that zombie.

Chalmers, who’s also famous for conceptually parsing consciousness research according to a language of “hard” and “easy” problems, segues into talking about what he calls the meta-problem of consciousness (find his 2018 paper bearing that title here). It’s interesting stuff, but it would have been nice had Chalmers done more to clear up what I take to be a misunderstanding underlying Carroll’s question. A question that, most charitably interpreted, amounts to the following worry: if zombies are conceivable, then I might be one.

Again, I’m certain that Carroll does not doubt that he himself is conscious. So the worry can’t be too dire. Perhaps philosophically unsettling would be the way to put it. For some people, though, the worry may run deeper than that.

To be clear, there are plenty of folks who characterize consciousness as illusory. But they don’t mean by this that consciousness doesn’t exist. They mean, rather, that consciousness isn’t what we tend to think it is—that it’s not, for example, some coherently unified thing seated “right behind the eyes” and under the executive control of another singular thing called a self. (See, for example, Susan Blackmore’s explanation of what she means by “illusion” in this context.) Indeed, most (maybe all) philosophers who get called out for denying the existence of consciousness (like Daniel Dennett) deny no such thing.

But maybe some people really are worried about the literal nonexistence of consciousness. I’ve certainly heard folks say that and not budge when questioned. I’ve always assumed they’re really just overstating—in a fit of heightened reductionist enthusiasm—something like the above-described “illusion” position. Or are just trying to be interesting. But, as impossible as it sounds, maybe some people really have confused themselves into believing that they might be no more conscious than a pile of laundry—might not actually experience the pain of a stubbed toe or the warmth of freshly baked pizza or the sadness of love lost.

That worry is obviously misguided. Interestingly, the zombie experiment demonstrates this when properly understood. It’s true that c-Carroll (for “conscious Carroll”) and z-Carroll both wrote poetry and cried at WALL-E. But there are also many things that c-Carroll did that z-Carroll did not do. For example, c-Carrol pondered the way his poetry’s words looked on the page, sounded in the air, felt in the mouth, resonated in the soul (so to speak). Z-Carroll definitely did none of this.

This suggests a way for you (not me!) to tell whether you are c-You or z-You. Just answer this short list of questions silently, in your head, making a mental note of your answers.

Are you experiencing these words right now? For example, do you see or hear them?

Are you experiencing your environment? For example, do you feel a chair?

What is your full name?

Where were you born?

Have you ever dreamed while sleeping?

Now, review your mental notes. If you answered anything at all to any of these questions, you are not a zombie.

Here’s a more elaborate report.

In terms of externally visible behavior, z-You and c-You responded in exactly the same way to the test. You both sat silently and appeared to take in the questions. You both scratched an elbow at one point, cracked some knuckles, yawned, and even exhibited identical finer movements, like eye saccades. Finer still, your neurons fired in precisely the same way throughout.

But there was a whole set of activities engaged in by c-You that z-You did not engage in. Namely, c-You actually answered the above questions*. Z-You did nothing of the sort, because z-You—like the pile of laundry (that’s by now been messily spread about the room)—has no perception, no experience, no mind, no imagination, no consciousness.

(*Alternatively, c-You might have silently refused to contemplate the test due to finding it silly. Understandable. But I can’t be sure, because I can’t read c-You’s mind.)

The upshot is that c-You engages in activities that z-You cannot, ones that are invisible to the outside world. Which is to say that if you silently answered “yes” to ever having dreamed, then you’re not a zombie; and if you silently answered “no” to ever having dreamed, then you’re not a zombie. Zombies can’t silently answer questions. Carroll was incorrect, then, when he said that “all the things I say and do are exactly what a zombie would say and do.”

Now, you might find it inconceivable that z-You could behave and have firing neurons just like c-You, yet not be conscious. For example, you might, on contemplating the zombie thought experiment, conclude that the statements “your neurons fired the same way” and “you experienced the same mental content” denote the exact same event, rather than two distinct events. This brings me back to my aforementioned assumption that Carroll’s real aim here is to emphasize that very point. That is: since he is conscious, and since he and his zombie counterpart are physically and behaviorally identical, zombies must be inconceivable. That’s fair, but, to my continued puzzlement, he then takes things a step further: if zombies do turn out to be conceivable, how can I be sure I’m not one?

The air with which he expresses that question varies depending on his audience and interlocutor. That’s only natural, but does make it all the more difficult to nail down exactly what argumentative work the question is meant to perform. This isn’t about uncovering Carroll’s motivation, but rather what he actually means by the question. How else to address it? That in mind, I’d like to examine the question one last time in light of my comments so far.

I must, however, resist the temptation to speculate any more than I already have about what Carroll is thinking (I can’t read minds!). I may be completely off the mark as it is. So, I will take his question as close to face value as I can manage. That is, as one to which Carroll thinks an answer is owed by those (like Chalmers) who find zombies conceivable. If Carroll is right, this would mean that Chalmers must either commit to being unsure of whether he himself is a zombie, or he must give up the conceivability of zombies—which is to say, again, that the question may be meant to shake Chalmers free of a conceptual mistake.

I should emphasize, however, that Carroll says in the intro to the podcast that his aim is neither to argue with nor to “convince David Chalmers in real-time that he’s wrong. But rather to let you the listeners hear what his perspective is on these issues.” Carroll effortlessly fulfills this promise.

This, then, is what I consider to be the most charitable interpretation of Carroll’s question: a good-faith expression of what he takes to be an unacceptable implication of buying into the zombie experiment. If so, this must come down to a misunderstanding. Maybe one as simple as failing (or declining) to play along with the experiment’s request to acknowledge—at least conceptually or semantically—a distinction between internal activities (e.g., evaluating the taste and feel of the wine) and external activities (e.g., saying, “This wine’ll do fine”). Carroll, rather, seems to only acknowledge the latter in his way of talking about both zombie and human behavior. This may be why he keeps calling zombies liars (e.g., in his book and on Harris’s podcast). I don’t see how an unconscious zombie could lie any more than a broken clock could.

Maybe, then, the disconnect is the following. The experiment explicitly asks us to imagine unconscious entities—i.e., zombies—behaving like us, while Carroll seems to be imagining conscious entities—i.e., us—behaving like zombies, and somehow manages to get out of this the unwarranted question: How do I know I’m conscious? Or (perhaps) more precisely: How is the zombie thought experiment not somehow internally incoherent?

I’m not sure if that’s a fair account of the disconnect, and I think I’ve already violated my no-more-speculation rule. So I’ll wrap things up by summarizing why I don’t think Carroll’s question follows from the experiment.

The zombie thought experiment explicitly asks c-You, and not z-You, to conduct the zombie thought experiment. Why so? Because z-You cannot imagine zombies. It’s not incoherent, then, for the experimenter to be certain of not being a zombie. On the contrary! The zombie thought experiment requires the imagination of a conscious non-zombie in order to exist. There is no thought experiment in a world without thoughts.

Really, then, this should be test enough: if you can even try to conceive of zombies, you are not a zombie.


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