Does anyone understand the modal argument against physicalism?

[Meeker]

Cause I sure as hell don't.

ETA: If it helps I should note I'm referring specifically to Kripke's

[anvi]

I don't like Kripke's modal argument against physicalism at all: too many counterfactuals and too much intuitionism for me.  But, here is how I understand the argument.

The core of physicalism is an identity theory, in which mental states = brain states ("pain" = "c-fibers firing").  Identities, Kripke claims, are necessary, and they are so in all possible worlds.   "H2O" = "water" in every possible world, provided that "H2O" and "water" are rigid designators that designate the same kinds of stuff across worlds.  So, if physicalism is true, the designators "pain" and "c-fibers firing" must designate the same individuals or kinds in all possible worlds, and in all possible worlds, they must be identical.

However, if we are considering tokens of "pain" and "c-fibers firing," these are not identical in all possible worlds.  I can conceive of a possible world in which this particular pain in my ankle occurs without the firing of "c-fibers," and I can also conceive of the firing of particular "c-fibers" without my experiencing this pain in my ankle.  If on the other hand, we consider types of "pain" and "c-fibers firing," these also break down in different possible worlds.  Worlds populated by zombies may exhibit "c-fibers firing" without the zombies feeling any "pain," and worlds populated by disembodied beings may see such beings experience "pain" without any "c-fibers" firing.  What we learn from these examples. Kripke claims, is that the only necessary identity that holds with regard to either a token of "pain" or the type "pain" is that "pain" feels like pain, while the occurance of "c-fibers" firing is not necessarily identical to "pain" in all possible worlds.

Therefore, if identity is necessary identity in all possible worlds, and if mental states are not identical to brain states in all possible worlds, then physicalism, which posits the identity of brain states and mental states, is false.  

Now, I think Kripke's argument fails miserably for lots of reasons.  But that's how I understand it.