David Lewis, Anesthesia by Genocide, and a Materialistic Trilemma
In his famous 1980 essay, "Mad Pain and Martian Pain", David Lewis tries to thread the needle between a flat-footed functionalism and a flat-footed neural-state identity theory about the mental. Flat-footed neural-state identity theory equates mental states, like being in pain, with possession of particular neural states. Thus, counterintuitively, it implies that beings who are behaviorally similar but internally very different, such as (hypothetically) Martians, can't feel pain. Flat-footed functionalism equates mental states with causal/functional roles. Being in pain, on such a view, is just being in a state that is caused by things like tissue stress and that tends to cause things like wincing, avoidance, and self-ascriptions of pain. This view, counterintuitively, implies the impossibility of "madmen" who feel pain for unusual reasons and have unusual reactions to it.
Lewis's solution is to say that some entity X is in pain if and only if X is in the state that occupies the causal role of pain for the "appropriate population". The "appropriate population", he says, might be (1.) us, since it's our term, (2.) a population that X belongs to, (3.) a population in which X is not exceptional, and (4.) a natural kind such as a species. In the normal case, all four criteria are met. In the Martian case, 2-4 are met though 1 is not, which is good enough. In the mad case 1, 2, and 4 are met though 3 is not, which is also good enough. Since mad Martian pain also seems possible, 2 and 4 alone will be sufficient for pain on Lewis's account.
Now the funny thing about these criteria is that they are all extrinsic or relational , and you might have thought that whether X is in pain or not should depend entirely on what is going on within X; you might have thought that pain would, in today's jargon, "supervene locally". The weirdness can be made vivid with further thought experiments. Criterion (3), for example, can be altered by genocide. Suppose that X is in a state that plays the causal role of pain for most of the population but the causal role of hunger for him and maybe a few others -- a "madman" case. On Lewis's account he will be experiencing pain. Now suppose that X is desperate to end his pain. On Lewis's account he might end his pain by perpetrating genocide upon all the non-mad people of the world. Voila, condition (3) flips, and X's pain has changed to hunger! This is anesthesia by genocide. We could similarly produce anesthesia by reproduction or speciation.
Real advocates of physical-state identity theory are hardly ever as flat-footed as those imagined by Lewis (as Lewis explicitly acknowledges). Like Lewis, they tend to embrace accounts on which to be in pain (or any other mental state) is to be in a state of a certain physical type, where the relevant physical type can vary between different types of being. What type of physical state is identical to what type of mental state, for beings of your type, then depends on facts about the particular causal or functional role of that state in members of your group or on the causal or functional history of that physical state in members of your group and/or in your own evolutionary or developmental past. Such type classifications are extrinsic or relational. Thus, such views have the bizarre consequences that flow from the denial of local supervenience. They allow anesthesia by genocide, or by speciation, or by hypothetical differences in past history that have no neural trace in the present.
We might thus see the mad pain-Martian pain issue as a trilemma in which each horn has bizarre consequences: Either accept the bizarre consequences of a strict functionalism (no mad pain), accept the bizarre consequences of neurobiological chauvinism (no Martian pain), or accept the bizarre consequences of denying local supervenience (anesthesia by genocide or speciation). Can a plausible materialist metaphysics dodge this trilemma? (I set aside hand-waving appeals to yet-to-be-identified intrinsic properties, a la John Searle.) I'd be very interested if you think you can point me to an example!
If all the options are bizarre, as I think, then something bizarre must be true. (Yes, dualism is also bizarre.) The problem is in figuring out which bizarre view to accept! If none of the various bizarre options merits credence, then crazyism follows.