Lewis presents an account of mental states which, he believes, can account for two problematic cases of supposed pain: the case of pain in a madman and the case of pain in a martian. The madman is in the state of pain, yet that state is not realizing the causal role R typically associated with the concept of pain. The martian, meanwhile, is in a different state than humans are in when they experience pain, yet this state all the same is realizing R just like the other physical state humans experience. How can the madman and the martian both be in pain? Lewis builds upon his previous theorizing in order to tackle the above question.
Since Lewis is really addressing two distinct questions, i.e., how can a madman feel pain, and, how can a Martian feel pain, there are two separate but intimately related arguments for each case. Lewis’s goal, of course, is to make the two accounts compatible with one another. A naïve identity theory (the thesis that mental states are brain states) will say that the madman is in pain but the martian is not. A naïve functionalist view, meanwhile, will say that the martian is in pain while the madman is not. Meanwhile, conjoining at random two theories which end up being compatible (which state that both are in pain) is ad hoc and therefore unjustifiable.
Lewis begins by presenting his proposal for what the concept of pain is: the concept just means ‘occupant of causal role R,’ where R is a conjunctive sentence of a state’s causal relations to stimuli, other mental states, and behavior. Importantly, Lewis thinks that the concept of pain (and thus the word ‘pain’) is non-rigid: it does not refer necessarily to the state(s) that it does, but only contingently. That is to say, if neural state N is the referent of the concept pain, then in some other world the referent is N2 rather than N. If this is right, then the concept can refer to different physical states in different possible worlds. And if this in turn is correct, then the concept can refer to different physical states in our own universe, since actualities (things which actually exist in our universe) are just a breed of possibilities.
Lewis thus has a solution to the problem of the martian: the concept pain still refers to the physical state which realizes causal role R in martians, because the concept is non-rigid and can have multiple referents. As such, martians are in pain in the sense that they have a state which realizes R, even though that state is very different from the one which realizes R in us humans. These respective states are, then, ‘pain-in-martians’ and ‘pain-in-humans.’
Under the above scheme, the madman is also in pain, albeit in a different sense than the martian. The madman is in pain because he is experiencing a state which typically realizes R in human beings. In him, (say, because his neural wiring is messed up), the state which typically realizes R in human beings does not in fact realize R in him. But he is still in the very same state which realizes R (pain) in the rest of us, despite it having different causes and effects in him. He is an exception to the definition of mental states under Lewis’s theory, and is handled by the typicality clause in Lewis’s definition of mental states: a mental state is the state which typically realizes causal role R in human beings, or whatever species under consideration.
If both the martian and the madman are in pain under this proposal, albeit in different senses, Lewis still needs to answer the question of the appropriate population the mental state realizer should be relativized to (in other words, which group do we plug in to ‘Y’ in ‘X is in pain just in case X is in the state which realizes the causal role of pain in Y’?). Lewis answers this question by providing criteria for determining the appropriate population: the population should either be us, since we developed the concept of pain, or otherwise a natural kind such as a species, or if we are trying to decide if X is in pain, it should be the group X is a member of, and/or a group where X is not exceptional.
The above criteria allows for vagueness, or indeterminate cases where it is not clear whether the person (or whatever sort of being under consideration) is in pain or not. Lewis gives an example: suppose there is a sub-population of human beings where the state that typically plays the causal role of pain in human beings causes thirstiness in the sub-population, and vice versa. If this is so, we might be tempted to think of the sub-population as a group of madmen, or of martians. There is no clear way of deciding in which sense they are in pain.
Lewis thinks that the above example parallels the problem of inverted spectra. Some people supposedly see red where we see green, and vice versa. Lewis thinks that such persons, when observing a patch of grass, see red in some sense, since they are in the state which typically realizes the causal role of seeing red in most people, whereas in some other sense they see green, since they are in the state which is typically associated with the causal role of seeing green in people with inverted spectra.
Lewis acknowledges that there is a case that his theory cannot handle: the case of a mad martian who is unique (i.e., who does not belong to any population). If such a being were possible and could experience pain, that pain would defy either sense of ‘pain’ established in Lewis’s account. As such, Lewis simply denies that such a case is possible. He says, rather, that it borrows elements from possible cases without itself being possible.
Finally, Lewis responds to a potential objection regarding qualia. The objection states that Lewis’s account fails entirely to describe the ‘what it feels like,’ or phenomenal aspect of pain. Pain is a certain feeling, regardless of what causal role it plays. Lewis responds by saying that he agrees with the objection, to a limited extent. The causal role R described by the concept pain is both the causal role and the feeling. So if a state is ‘pain-in-humans’ (the realization of R in typical human beings), then having the state is feeling it.
Argument: L-theory (a term I’m using to denote Lewis’s theory presented in the paper) can account for martian pain and madman pain, a prerequisite to any theory of mind.
1. L-theory can account for martian pain.
S1. A theory which accounts for martian pain has to allow for martians to experience pain (in some importance sense of ‘pain’).
S2. L-theory allows for a creature to be in pain just in case it is experiencing a state which realizes R (the causal role defined by the concept ‘pain’) or the state which typically realizes. R in the appropriate population.
S3. Under L-theory, martians experience a state which realizes R.
S4. Therefore, L-theory can account for martian pain. [S1-S3]
2. L-theory can account for madman pain.
S1. A theory which accounts for madman pain has to allow for madmen to experience pain (in some importance sense of ‘pain’).
S2. L-theory allows for a creature to be in pain just in case it is experiencing a state which realizesR (the causal role defined by the concept ‘pain’) or the state which typically realizesR in the appropriate population.
S3. Under L-theory, madmen experience the state which typically realizes R in human beings.
S4. Therefore, L-theory can account for madmen pain. [S1-S3]
3. Therefore, L-theory can account for both martian and madman pain. [1-2]