Hail Mary
Why physicalists should love Mary and what they should say about her
Hail Mary,
Full of science,
knowledge is with thee.
Blessed art thou among scientists,
and blessed are the books
in thy room, Wowzers!
Hey Mary,
exit the room,
and check out this apple now,
and tell us
did you already know
how it’d look?
I love thought experiments, and the Mary thought experiment at the heart of Frank Jackson’s “knowledge argument” against physicalism is one of my all-time favorites, even though it utterly fails to disprove physicalism. Unfortunately, most physicalists bypass the best response to the knowledge argument, which I am happy to share here. In case you’d like a re-cap of the argument itself, presented in comic form, scroll down to the end of this article. Then scroll back up to enjoy this response.
Why It Follows from Physicalism that Mary Can Know What Red Is Like
Frank Jackson’s “Mary’s Room” thought experiment is a fan favorite for poking holes in physicalism. Mary, the genius scientist, knows every physical fact about color vision—wavelengths, neurons, the lot—but has never seen color, locked in her black-and-white world. When she steps out and sees red, does she learn something new? Jackson says yes, hinting physicalism misses the mental magic of “what it’s like.” I say no. Mary can know what it’s like (KWIL) to see red in advance, and this squares perfectly with physicalism’s bedrock: nothing mental is fundamental. Better yet, we can snap the supposed link between knowing and experiencing wide open. Here’s how.
Knowing Without Seeing: Red Flags and Missing Blues
Start with a warm-up. You’ve never seen a Japanese flag. Someone says, “It’s a white rectangle with a red circle in the center.” Get the concepts—white, red, circle, rectangle—and you can KWIL seeing it, no flag required. The flag’s a complex; its parts (redness, circularity) match the description’s parts (concepts of red, circle). You don’t need to see it to know.
David Hume beat me to this punch centuries ago with his “missing shade of blue.” Imagine you’ve seen every shade of blue except one. Line them up, spot the gap, and you can KWIL that missing shade without ever seeing it. Why? Your mind builds it from what you’ve got—blues are complexes, not atomic mysteries. Like the flag, a good description or pattern fills the blank. KWIL doesn’t demand experience.
Judging Without Feeling: Crane’s Twist
Philosopher Tim Crane adds another angle. I can know what it’s like to be in pain—say, a throbbing headache—and use that to judge correctly that I’m not in pain right now. No ache, no wince. Here’s the kicker: I’m deploying KWIL pain to confirm its absence, not its presence. Knowing what an experience is like isn’t tethered to having it—it’s a tool I can wield either way. This cuts deeper into the Jacksonian hunch: KWIL isn’t literally made out of experiences.
Physicalism’s Core: Complexes, Not Fundamentals
Physicalism is a giant pain to fully define. But it has a clear core: Nothing mental is fundamental. Seeing red isn’t a basic spark—it’s a complex of non-mental simples (neurons, photons, whatever). How the complex emerges—reduction, realization—doesn’t matter here; what matters is it’s built, not bedrock. If that’s true, a complete physical description of seeing red should let someone KWIL it, just like the flag or Hume’s blue. Enter Mary.
She’s got the full playbook—every physical fact about red vision in a Big Long Accurate Description (BLAD). We don’t have it, yet—we’re stuck in the present, waiting for the future. But futuristic Mary’s got the goods: a scientific theory that’s as good as it gets. If seeing red is a complex, the BLAD spells it out—structure, parts, assembly—and she grasps it. She knows what it’s like before stepping outside.
Swamp Mary: The Knockout Punch
Still skeptical? Meet Swamp Mary (her adventures chronicled in my paper “Swamp Mary’s Revenge,” in Philosophical Studies). She’s a bolt-from-the-blue duplicate of post-red-seeing Mary—same brain, same wiring—but she’s never seen red herself. Lightning zaps her into existence, Swamp-style, making her molecule-for-molecule identical to Mary after the apple encounter. Does Swamp Mary KWIL seeing red? Yes—her head’s packed with the same neural setup, the same whatever Mary definitely has post-apple. Swampy’s never experienced red, but she knows it cold. The link between KWIL and “having seen” snaps clean off.
Why the Knowledge Argument Fails
Jackson bets Mary learns something new when she sees red—that KWIL can’t come from physical facts alone. Too many physicalists shrug and agree, but they shouldn’t. If seeing red is a complex, and physicalism says it is, then a perfect description (Mary’s BLAD) delivers KWIL upfront. Hume’s blue, Crane’s pain check, and Swamp Mary all hammer it home: knowing what it’s like isn’t chained to experiencing it. Jackson’s “new knowledge” claim smuggles in a non-physicalist ghost—some mental extra beyond the complex. That’s not a gap in physicalism; it’s a rejection of it.
The Payoff
Mary doesn’t need to see red to know it. She’s got the manual, and she’s read it. Physicalism stands tall: mental states are built, describable, knowable—no fundamental woo required. Next time someone waves Mary’s Room at you, tell them she’s not stumped—she’s just better prepared than we’ll ever be.
And now, comics:
I am a physicalist, at least in this sense: I believe (though I know I cannot prove) that the mind is a result of purely physical processes occurring within the brain, and, furthermore, that how this works could, at least in principle, be explained within our current understanding of the physical world, requiring physics no more fundamental than is needed for biochemistry.
Nevertheless, I suppose that Mary will learn something new when she sees red, and I do not do so with a shrug - on the contrary, I feel that asserting she would know what it's like prior to seeing red requires assuming the brain has certain capabilities that are implausible and lacking empirical evidence.
Physicalism (or at least my flavor of it) effectively implies that coming to know what it is like to see red requires some physical change in the brain - changes which can be traced back to the stimulation of red receptors in the retina, and while the specifics are probably different in detail from one person to the next, are similar at some appropriate level of functional abstraction. Under the premise that Mary knows every relevant physical fact, we must assume that she knows (e.g. through scanning and modeling) exactly what changes will occur when she sees any particular red-containing scene, but there's no reason to believe she has the ability to bring about those changes through mental effort alone - and, according to the physicalist premise, if the changes do not occur, she will not know what seeing red is like. Consequently, any physicalist who claims that Mary will learn from her studies what seeing red is like, carries the burden of providing some justification for assuming that she can use her knowledge to bring about the requisite neurophysical changes.
I am saying that there is no good evidence, in either personal experience or neuroscience, that we would be able to bring about the necessary changes at the neuron and synapse level through mental effort alone, unaided by tools - but what about the examples you give? Even if Hume was right, interpolating between very similar experiences seems considerably less demanding than synthesizing them from scratch, and it seems (at least to me) unlikely that someone who had only seen blues and greens could come to know what seeing orange and red is like, yet Mary is expected to do even more with less. Similarly, if you do not already know what red is like, it seems unlikely that you can know, except in abstract terms, what the Japanese flag looks like. As for Crane's argument, if you had never experienced pain (and there are some people who do not), I suppose you could, on hearing other peoples' talk about pain, deduce that they have experienced something you have not, but that would not tell you what it is like. Finally, Swamp Mary is not coming to know what seeing red is like through her own mental effort.
How, then, can I dismiss the threat to physicalism that Jackson's thought experiment allegedly poses? Basically, it is for the same reason as I object to the thesis that Mary will know what seeing red is like prior to doing so: no matter what she knows, she cannot bring about the requisite physical changes. We understand well enough how various complex and indisputably physical systems work - the weather and biological reproduction, for example - yet no-one expects that learning this knowledge will create a thunderstorm or get you pregnant. Why should anyone think the mind is any different? I think Churchland answered this question in an unfortunately hard-to-find article with the title "Knowing Qualia: a Reply to Jackson" way back in 1989: the argument equivocates over the meaning of the verb "to know." While we talk of knowing what it is like, it is not knowledge in the sense of having a justified belief in certain propositions (if it were, how come we cannot say what these propositions are? And why would anyone assume that Mary could not learn them from her studies?)
Here's an analogy which might serve as an intuition pump for this view: suppose a safe-cracker has broken into a bank one night, and she has X Rays of the vault, so detailed that she can deduce the combination from them. Nevertheless, she still cannot open it if it contains a well-designed time lock that is preventing anyone from doing so until tomorrow morning. She has all the relevant physical knowledge, but not the means to use it.
Regarding physicalism, my take has long been that the setup to the thought experiment assumes its purported conclusion, non-physicalism, by implying that Mary can have all the physical information and yet still subsequently learn something new. If we remove that assumption, then it becomes an assertion that Mary can have all the information but learn new information after seeing red, a contradiction.
The question of whether it's possible for her to KWIL with all the possible third party information is a more interesting question. Swamp Mary is a nice move! It seems similar to Daniel Dennett's RoboMary putting her brain in the same state as if she'd seen red. In both cases, KWIL is achieved without the experience of seeing being in the causal history. Nice!
I also think it's worth noting that even a Mary only with contemporary knowledge could guess a lot about what she'd see. She'd know that reds blend into oranges, which blend into yellows, greens, cyans, blues, etc. She'd expect red to jump out more than green or blue. And she'd be expecting it as a commonality for roses, blood, and ripe strawberries and a strong distinction from the sky, grass, or ripe bananas.
I think most of our intuition about how she'd react seems more based on how we'd react. But without her knowledge, out intuition doesn't seem useful here.