Not necessarily. By hypothesis, Mary has complete knowledge of all the physical facts. That includes the knowledge of how to cause her optical/neural pathways to be excited in the particular way that corresponds to "seeing red" even without the corresponding external stimuli being present. So she could have caused herself to see red. (We might describe this in more conventional language as Mary imagining seeing red, and her complete knowledge of all physical facts giving her powers of imagination far in excess of those of ordinary humans.)

Yes, Mary would have the knowledge of how to perform surgery on herself to stimulate her brain or optic nerves in the right way to generate the necessary experience. I don't think that's within bounds of the thought experiment though.

The thought experiment is testing our intuition about whether third-party objective facts, as found in science, can suffice to describe even subjective experiences like redness. If you need to actually stimulate the experience to gain that understanding, then that either proves that third-party objective facts are not sufficient, or it proves that such experiences are not knowledge.

That's one standard response: knowledge and ability are not equivalent. Someone who had their legs amputated ostensibly knows how to walk but lacks the ability to do so. In principle, no amount of knowledge by itself will make you walk, but knowledge can give you back the ability to walk, ie. a gene therapy with stem cells to grow back limbs, prosthetics, etc.

So the idea is that experiencing redness is not knowledge but an ability you acquire by exposure to colours: https://en.wikipedia.org/wiki/Knowledge_argument#Ability_hyp...

Why not? The point of the thought experiment is supposed to be to prove that Mary learns something she didn't know before when she sees the apple. But if the premises of the thought experiment give her the ability to give herself that knowledge beforehand, that undermines the conclusion.

> The thought experiment is testing our intuition about whether third-party objective facts, as found in science, can suffice to describe even subjective experiences like redness.

No, it's not. It's claiming to prove that subjective experience cannot be physical, i.e., that physicalism is false. It is making use of a common intuition that subjective experiences are fundamentally different from "third-party objective facts" (Dennett calls it an "intuition pump" for this reason), but that is not a proof of anything, it's just a circular argument, assuming what it purports to prove.

> If you need to actually stimulate the experience to gain that understanding, then that either proves that third-party objective facts are not sufficient, or it proves that such experiences are not knowledge.

Neither one of these refutes physicalism, however, since the process of giving yourself the required stimulus based on your knowledge is a physical process--at least, there is nothing in the premises of the thought experiment that rules that out.

No it doesn't, it's called the Knowledge Argument because the knowledge of redness purportedly does not deductively follow from the knowledge of any amount of physical facts. It requires non-deductive steps, like surgery and brain stimulation in this case.

You might as well say that Mary could construct a red LED and gain the knowledge of redness that way, because that too is a physical process. That's obviously out of bounds, and surgery is the same type of invalid move.

Edit:

> Neither one of these refutes physicalism, however, since the process of giving yourself the required stimulus based on your knowledge is a physical process--at least, there is nothing in the premises of the thought experiment that rules that out.

Under dualism, physical processes are correlated with non-physical qualia, so triggering the physical process in a roundabout way is not a valid move here. The question being tested is whether knowledge of redness follows from knowledge of all other physical facts, not whether knowledge of redness follows from triggering a physical process because we already know that does.

Which is just a bare assertion, not an argument. Nothing in the premises of Jackson's thought experiment entails this or requires it. To a physicalist, knowledge of redness is knowledge of a physical fact, so this statement is simply false.

> surgery is the same type of invalid move

If you're hung up on the word "surgery", then eliminate it: it's just as possible that Mary could simply imagine, based on her complete knowledge of all physical facts, what it would be like for her to see red, without having to do any surgery at all. You might say this is implausible, but so is the premise that Mary has complete knowledge of all physical facts. But that's the premise of the thought experiment.

> Under dualism

Jackson's thought experiment does not prove that dualism must be true. Nor can you assume that dualism is true as a premise, since that is just a circular argument. A physicalist, like me, simply does not think dualism is true, and it is no argument against that to show what would be the case if dualism were true. You need to argue, from premises that don't already contain dualism, that dualism must be true. Jackson didn't do that. Nor do any of the other arguments described in the article, including the Knowledge Argument.

"Imagining" is not the same as surgery. Do you agree or disagree that Mary constructing a red LED to experience redness is not a legitimate response to the Knowledge Argument?

It depends on what you think the Knowledge Argument is supposed to prove.

If you think the Knowledge Argument is supposed to prove that physicalism is false, then I disagree. To refute physicalism, it is pointless to say that some physical processes are not "knowledge". Even if the physicalist agrees, that in no way refutes physicalism. The physicalist simply says that the processes that Mary induces in her brain by constructing a red LED (or doing surgery on her optic nerve, or just imagining what seeing red would be like), even if they are not "knowledge", are still physical processes, and that those physical processes are Mary's experience of seeing red. Nothing in the Knowledge Argument refutes that.

If you think the Knowledge Argument is supposed to prove that there are physical things that aren't "knowledge" (whether or not there are also non physical things is then a separate question), then I neither agree nor disagree as a matter of physics, since what we use the word "knowledge" to refer to is a matter of definitions of words, not physics. I have no problem with accepting your definition of the word "knowledge" for purposes of this discussion. I just don't think accepting that definition says anything about whether or not physicalism is true.

This isn't relevant. Nobody is really disputing that physical processes cause qualia. You don't see the redness of an apple until the apple actually enters your visual field. Cause=physical event, result=qualia. Even dualists agree with this.

Therefore the Knowledge Argument is not about whether any physical process is causally linked to qualia, it's specifically about whether qualia can be deduced only from factual knowledge, because under physicalism, all facts are physical facts.

If physicalism is true, then all facts are third-party objective physical facts. Therefore quale are physical facts. If given an understanding of all physical facts, then Mary should be able to deduce her quale. If she cannot deduce her quale (or it seems very implausible), then either some mistake has been made or physicalism is false. Where is the mistake?

You cannot answer this question by saying her knowledge gives her the ability to trigger the usual physical process to cause qualia, because the question is about the deduction. The only outcomes of this thought experiment are one of: 1. denying that physicalism is true, or 2. denying that quale are facts, 3. accepting that Mary would be able to deduce her quale, 4. concluding that the thought experiment contains some inherent contradiction.

No, it's not, it's part of the explicit premise of the thought experiment.

It's not a conclusion; to a physicalist, it's part of the premise, since knowledge of her qualia is "factual knowledge". You can't just declare by fiat that this is false; that's a circular argument, as I have already said multiple times.

> you have provided no proof

The burden of proof is on you, not me; if you want to make an argument based on the claim that knowledge of qualia is not "factual knowledge", then you have to prove that it isn't. You can't just declare it by fiat.

I understand that non-physicalists, like you and Jackson, believe that knowledge of qualia is not "factual knowledge". But physicalists, like me and Dennett, disagree. Given that disagreement, you can't make your belief a premise of an argument and expect me to accept it. All you're doing is restating your position. Fine, I understand that's your position. But it's not the same as giving an actual argument for it based on premises that I must accept.

No, you left out one:

5. Denying that there is any difference, from the standpoint of testing whether physicalism is true or false, between Mary "deducing" her quale and Mary "triggering" her quale with a red LED or surgery or by imagining.

Basically you are trying to make an argument by redefining words to make arbitrary distinctions that don't have any bearing on whether physicalism is true or false. First you want to limit what Mary has to "knowledge", but nothing in physicalism requires all physical things to be "knowledge" or requires that all physical things must be reachable from "knowledge". Then you want to limit Mary to only having "factual knowledge", but nothing in physicalism requires all physical things to be "factual knowledge" or to be reachable from "factual knowledge". Then you want to limit Mary to only having "third-party objective physical facts", but nothing in physicalism requires that all physical things are "third-party objective physical facts". Then you want to limit Mary to only be able to "deduce" her quale instead of triggering it or imagining it, but nothing in physicalism requires someone to have to "deduce" their quale in order to know what it is like without having been subjected to the normal external stimulus (in this case the image of a red apple on Mary's retina) that would trigger it. Physicalism simply does not make or require any of these distinctions you keep making.

The claim of physicalism is simple: Mary's quale "seeing red" is a physical thing. (Or "process", or "fact", or whatever you want to call it; the substance of the claim remains the same.) The only way to disprove that would be to show that her quale being a physical thing somehow leads to a contradiction. The Knowledge Argument does not do that. It does not even claim to do that. It simply constructs a straw man version of "physicalism" that is vulnerable to refutation by this particular argument even though actual physicalism is not.

When you look at what physicalism actually says, the so-called Knowledge Argument boils down to a bare assertion. The premise is that Mary has all the physical information. The claim is that she learns something when she sees the red apple, implying that she did not have the information about what it would be like to see the apple beforehand, when she had all the physical information. But the claim simply does not follow from the premise; it is a non sequitur. The only way to get to the claim from the premise is to add an additional premise that having all the physical information does not include having the information about what it is like to see the red apple. But of course that is just arguing in a circle; it is simply assuming that physicalism is false in order to "prove" that physicalism is false.

The whole point of physicalism is that having all the physical information is having all the information, so it includes knowing what it is like to see red, to taste wine, to feel the sun shining on you, etc., etc., etc.. So any thought experiment that claims that Mary, who has all the physical information, will learn something when she sees the red apple is simply based on a false premise according to physicalism. Which makes it obvious that no such argument can refute physicalism.

No, this is a well studied problem by thousands of philosophers over 40 years. Maybe reconsider your position that you understand this argument better than they do. I don't see any benefit in continuing this discussion at this point.

Ah, the old argument from authority. Sorry, not buying it.

> I don't see any benefit in continuing this discussion at this point.

Neither do I.

Having knowledge and actually doing it are trivially different things (try sex: knowledge of sex and doing the deed are vastly different things).

Now, if she exercises her knowledge by stimulating her optic nerves, that's the same as seeing the apple. But that's not just "reading about it", it's a physical act of doing!

This is a circular argument. You are arguing that Mary does not know what it is like to see red by assuming that she does not know what it is like to see red.

> if she exercises her knowledge by stimulating her optic nerves

Which, since she has complete knowledge of all physical facts, she can do. The thought experiment does not say she "reads about" color vision. It says she has complete knowledge of all physical facts. The article referenced here does not do a good job of describing Jackson's actual thought experiment, but Jackson's actual thought experiment is based on the (extremely strong and unrealistic) premise I have just stated. It has to be, because any weaker premise cannot possibly ground a conclusion that Mary learns something non-physical when she sees the apple. Just saying that Mary knows everything that can be learned by reading about things is not enough; there are many physical facts that cannot be learned by reading.

I realize Jackson is a professor and I'm not, and that this is a well known thought experiment, but I deem it so obviously invalid -- or rather, based on false premises -- that it's not worth considering.

Of course I'm basing my opinion on TFA, but reading Wikipedia's summary I see other philosophers have poked holes on this experiment and even agree with me the premises are false.

P.S. defining "knowing everything there is to know" the same as "physically experiencing" is circular reasoning, and I definitely disagree with this premise.

Your premise only seems plausible as you state it because you are not actually considering the implications of knowing everything there is to know about sex. But knowing everything there is to know about sex includes knowing what it is like to have sex. So the real premise you should be stating is that it is not possible to know everything there is to know about sex if you have not actually had sex.

And, as you appear to realize, making the appropriate adjustment in how the premise of Jackson's Mary thought experiment is interpreted makes it obvious that the argument is invalid--and I agree that it is.

It sounds too much of the old Maths joke: "let's consider a spherical cow."

This complaint should be addressed, not to me, but to Jackson, whose premise for the thought experiment gives Mary such "superpowers". I agree that such a premise is completely unrealistic and invalidates the thought experiment. Unfortunately many philosophers do not appear to have gotten the memo.

But to summarize for you: it's not true that Mary "knew all" just by reading about the color red. The premise is false. She lacked one bit of information: to have her optic nerve stimulated by a red apple. Note this isn't metaphysical, it's physical; such stimulation can be physically measured and she lacked this experience, which -- to emphasize the point -- is entirely physical.

Again, I think the thought experiment can be probably reframed in order to fix this flaw, but Mary and her red apple as currently stated are not convincing of any duality.

If Mary knows everything there is to know about colour vision, that includes knowing the experience of seeing the colour red.

If she doesn't know that, it simply contradicts the premise.

If so, duly noted.