The order in which I have ranked Nagel, Kripke and Jackson is in no way due to the order in which I have read them, although it would appear so. I rank their arguments against physicalism in just that order, from weakest to strongest. First, Nagel’s paper explicitly says that it is not an argument against physicalism. It is merely an opening up of dialogue about the possible completeness of physics, and in it Nagel argues there will always be a certain subjective realm about which science is unable to talk about exhaustively. The subjective/objective distinction, while carrying with it the possibility of radical Pyrrhonistic skepticism, is usually interpreted to be a fairly mild explanation of why physicalism as a project of ‘scientism’ is inadequate and cannot know inner subjective experiences. Yet it is “hard to see any objection to physicalism here,” as Jackson says, since it does not explain why physicalism would need to know exactly what-it-is-like for a foreign body to have subjectivity, or qualia. Hence Nagel’s argument, while provocative and persuasive, is the least offensive to physicalism.
Saul Kripke’s paper is a bit more of a challenge, yet due to the nature of the argument, which is based on whether the reader shares the conviction of the author or not, it is difficult to see that it delivers a decisive blow to physicalism. Jackson seems to make an empirical claim that if at least one person is unconvinced of the modal argument then it is doubtful. The modal argument, stripped of the creativity and imagination, says in one way or another that no amount of physical information about another person will tell you if they are conscious at all. But why should we think that’s true? The modal strategy says that if supervenience is not necessary, then physicalism is necessarily false. This is an acceptable strategy, yet by begging our intuitions to accept the anti-supervenience argument, it argues that supervenience fails in a number of imaginary cases. Zombies, for example, are said logically possibilities. But this isn’t a strong argument against the claim that everything that is physically identical is “mentally” identical by definition. Or everything that is physically identical must give rise to the same aesthetic qualities. The argument has no other substance than this one claim, provided by numerous examples. While I share Kripke’s notion prima facie, I doubt its validity not on the basis of Kripke’s unmet burden to shake my counter-intuitions into conformity, but based on the idea that it is not logically possible for zombies, et al, to exist. That is, the first premise in the argument in syllogistic form.
Frank Jackson’s argument is the most challenging, particularly since it is the most debatable. Nagel and Kripke are less debatable since either one shares the intuition or one doesn’t. Yet as I understand Mary’s Problem in particular, she is either lacking in relevant knowledge, or has not made the proper use of it. Assume, as Frank Jackson setup the problem, Mary really does know everything about the physics of light and the optics of color, etc. In that case she must know how the deep structures of the brain respond to information from the peripheral sense organs, such that these brain mechanisms give rise to — qualia! If Jackson were to argue that Mary still doesn’t know how brain mechanisms give rise to qualia, it is based on his own subjective view that Frank Jackson does not know how brain mechanisms give rise to qualia. His argument seems to presuppose that Mary would know this, except he pulls this ground out from under us in arguing that she in fact doesn’t know what-it-is-like for qualia to be experienced in, for example, myself. Yet physicalism doesn’t need to provide an account of this for it to be true.
While I believe Jackson ’s Mary Problem is the most debatable, it is also the least valid, and hence not the stronger argument. While I tend to share Nagel’s convictions, I don’t believe Bat Problem even poses a considerable challenge to physicalism. Thus Jackson ’s argument remains the ‘least weak’, while Kripke’s is not actually an argument.