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Frank Jackson’s classic thought-experiment runs like this:

Mary is the world’s leading expert on the neurophysiology of vision, yet she has lived her entire life in a black-and-white room. Inside that room she learns every physical fact about color: electromagnetic wavelengths, retinal transduction, visual-cortex circuitry, the works. One day she steps outside and sees a red tomato. At that instant she acquires something she never had before: the subjective feel—the quale—of red.

From this, dualists argue:

Before leaving the room Mary already knew all physical facts. After leaving, she learns something new: what red looks/feels like. Therefore the complete story of conscious experience cannot be captured by physical facts alone; qualia must be non-physical “extra ingredients.”

The Pressure Point

But notice what actually happens when Mary does see the tomato: photons in the 620-740 nm range strike her retina, cascade through the lateral geniculate nucleus, and fire up color-responsive neurons in V1, V2, and V4. In other words, her brand-new “knowledge” just is a previously unavailable physical data stream coursing through her nervous system. Nothing supernatural appears on the scene; a new pattern of spikes does.

A Single Question for the Dualist

If Mary's first encounter with red is nothing more than the brain’s reception and processing of a novel, measurable stream of physical information, what grounds are left for claiming that the resulting quale is a non-physical property, over and above that complex physical event itself?"

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  • that seems to be missing the point. Mary already knew in the room that upon encountering a tomato, "photons in the 620-740 nm range strike her retina, cascade through the lateral geniculate nucleus, and fire up color-responsive neurons in V1, V2, and V4" Still that did not allow her to know what redness looks like. Also Dualism does not imply supernaturalism, only non-monism (non-physical-monism). Commented Jun 17, 2025 at 16:43
  • ou are absolutely right: Mary already knew the propositional fact that "photons will strike her retina and fire up neurons in V1, V2, and V4." My question, however, hinges on a crucial distinction: the difference between knowing a description of a physical event and the physical event actually happening in your own system. Before leaving the room, Mary knew the facts about the process, like someone who has memorized the entire chemical formula for chocolate and the neurobiology of taste. After she leaves the room and sees red, a new physical process occurs in her brain for the first time Commented Jun 17, 2025 at 16:51
  • So my question stands: you are pointing to a gap between her prior knowledge and her new experience. I am pointing to a new physical event and claiming that event fills the gap. What grounds are there for insisting we need to posit a second, non-physical thing on top of it? Commented Jun 17, 2025 at 16:56
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    Leaving aside the aloofness of the scenario (in this thought experiment Mary must not be alowed to see her own hands or lips, cut herself, or have her periods), there's an assumption hidden, which is that red qualia are not fully knowable theoretically, as second-order knowledge, and that they can only be experienced directly. IOW, that qualia are inefable is assumed but not stated explicitly. So I think the thought experiment is circular: it starts from its own conclusion: there is something in experience that is beyond theoretical knowledge. Commented Jun 17, 2025 at 17:37
  • I don't think it's about the complex physical event itself, but more that you're there for it, right? We can imagine that the event can occur with a pZombie but no one is "there for it", on the inside. So qualia is trying to get at the difference between a pZombie seeing red and whatever we are seeing red. It's tough squeezing science in there. Physicalism is fine though, just with a limit on science. Commented Jun 17, 2025 at 19:30

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The obvious point is that Mary doesn't experience a stream of physical information. Mary experiences a color. We can experience color without knowing anything about light (and its interaction with the eye and brain); we can know everything about light (and its interaction with the eye and brain) without experiencing color. But we don't understand how the physical processes of encountering photons translates into the subjective experience of perceiving color. That's the problematic here.

Say we have one of those old video arcade units where we can play PacMan. We experience that as a game where we use a joystick to guide PacMan around a course, eating dots and avoiding ghosts. But from the perspective of the arcade unit there is no PacMan, no ghosts, no dots, no joystick, no game… It's all just a set of mechanical and electrical signals being passed around, spraying electron beams across a treated glass surface. The game and characters exist on a different level of abstraction that the machine can't recognize. The game and characters also exist on a different level of abstraction from the physical processes of the brain and body. I don't know where that different level of abstraction comes from or arises, and no one else does either, but I don't see the sense in trying to pretend it doesn't exist. Note that this abstraction doesn't necessarily entail dualism; but it does call for explanation.

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  • I often wonder what my car 'sees' when it warns me that I'm straying from the lane or about to bump the car ahead. It represents the scene accurately in 3D somehow. It is no less useful for the fact that it's a mindless automaton. Autopilots can actually land a plane safely, they don't just give guidance. Commented Jun 19, 2025 at 1:49
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There was a fact missing from Mary's pre-experience knowledge: what it feels like to experience color. But this may be explained by the fact that others' attempts to communicate that subjective experience fall short.

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    There is evidence to suggest that certain colors trigger the release of dopamine and/or seratonin. So knowledge of the physics does not provide the full experience of viewing a color. Commented Jun 17, 2025 at 23:52
  • Can cardiologists repair their hearts by thinking about them? :-) Consider how brain surgery could be revolutionized. Yet, Jesus said, "Who, by thinking, can add one inch to his height?" (my point is that there is a difference between thinking and experiencing, or experiencing and knowledge. Else everyone would just read about food and starve to death.) Commented Jun 19, 2025 at 2:02
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If your point is (or includes) that Mary is the sort of thing which can be informed, and a physical process that happens to Mary's body is the sort of thing which can inform her, that's mind-body dualism and the dualist need not respond except to agree.

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The problem is that there is Mary in the room and Mary later outside the room. We can call them Mary1 and Mary2.

So what can Mary2 tell Mary1 about redness given that she made the experience, which Mary1 did not already know? If experience was a gain in knowledge, and knowledge in a monist physical universe can be captured by scientists in models and maths, then Mary2 gain of knowledge should enable her to tell Mary1 something Mary1 did not know yet.

It's not the fact that Mary starts seeing red outside the room that is the argument for dualism, it's the problem that whatever she has gained from seeing red cannot be translated into a physical description if the world.

Mary2 knows more than Mary1, yet all they know about physics is what brain activity happens, and so Mary1 and Mary2 have the same knowledge about the physical world, despite having some different knowledge. If they have a difference in knowledge, but that difference is not capturable in monist physics, monist physics cannot be complete.

This can be captured mathematically. Let knowledge K be all knowledge of Mary, with P being her knowledge that is physical monist and D any knowledge beyond monism. K, P, D > 0 (a) and P+D=K (b)

Then from the experiment we gather

K2 > K1 (c), but P2==P1 (d)

Therefore

     K1 < K2        from (c)
<=> P1+D1 < P2+D2   from (b)
<=> P1+D1 < P1+D2   from (d)
<=> D1 < D2         -P1
<=> 0 < D2 - D1     -D1
<=> 0 < D2          from (a)

D2 greater than zero means Mary Gaines non-monist knowledge, proving dualism, according to the argument.

The argument is not sound in the sense that it proves to both prior dualists and physical monists that dualism is true. But there is no new objection to it here.

If Mary's first encounter with red is nothing more than the brain’s reception and processing of a novel, measurable stream of physical information,

"Nothing more than" is hand-waving, this is not a physical description of experience of redness. There is a correlation or supervenience of the brain activity and the sensation, but that is insufficient to prove equality. Even if you say: "All life experience is just nothing more than brain waves", that is a mere claim, not a theory.

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My 2 cents ...
The physical of colors is that they are different wavelengths/frequencies of EM radiation. If you know the wavelengths of colors, you can order them; the result our familar rainbow.

However, given some colored strips, I don't think (not knowing the wavelengths), "anyone" would be able to put the colors in the order they appear in a rainbow. I know I won't be able to.

Relevance: Assuming wavelengths are the physical of color, and if the perception of color is purely physical, the inability to put colors in order is inexplicable.

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    Interesting argument but provably invalid. Color perception is not purely determined by wave length (e,g, there are contextual aspects); differences in perception don't map linearly to wavelengths. The inability to order colors just shows that perception is based on neural processing which is not the same as physical measurement. Commented Jun 17, 2025 at 20:59
  • @mudskipper said "Color perception is not purely determined by wavelength". I was working under the assumption that color is equivalent to wavelength physically (how else do we tell the colors apart?). Yet we're unable to order the colors wavelength-wise based on only the perception of color. Commented Jun 17, 2025 at 21:17
  • If a computer screen can only radiate varying quantities of three specific colors of red, green and blue, how do they show yellow? The eye sees a mixture of red and green and maps it somehow. But, yeah, nothing says most people know the rainbow without an example of one. I drive my car often, but I couldn't assemble one from a box of parts. Commented Jun 19, 2025 at 1:58
  • @ScottRowe I understand the predicament. Quite different with sound or, perhaps notes is the more accurate term. There is an acoustic paradox I recall having heard. Commented Jun 19, 2025 at 12:30
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This entire puzzle is nonsensical from the get go since when you talk about Mary’s knowledge, you’re already assuming that her conscious knowledge is some sort of entity that is fundamentally different from everything else in the universe, but that begs the question for dualism.

If consciousness is an emergent process like software is with respect to hardware, it is like asking “can the software process know what it feels like to do something?”

That question would be absurd since that is already assuming that it is meaningful to talk about qualia with respect to software. A software process is a process. It’s not an object in the first place. It’s not an extra piece of ontology out there in the world that is separate from the physical circuits generating it. So “qualia” with respect to software doesn’t actually exist.

The same applies here. If consciousness is an emergent physical process, which all evidence seems to suggest given the complete physical dependence of consciousness on the brain, then it makes no sense to talk about what the process “feels” or “knows”, since by definition it’s emergent. It’s like asking whether a car ride knows everything there is to know about the physical components of a car.

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