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?

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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: