The Modal Argument
As put forward by Kripke, and described in detail in [].
Kripke argues that, if there is an identity between mind and brain then this identity must be must be necessary, not merely contingent. For instance there is no world where Mark Twain is not Samuel Clemens and it makes no sense to imagine one. The identity between the two is necessary simply because the two names refer to the same person.
On the other hand, it does make sense to imagine a world where there are brains but no pain, and maybe vice versa. So these are not equivalent situations. Imagine, for instance, of a being who shares all our material properties, yet has no consciousness whatsoever. Such imagined creatures are referred to as Zombies. Conversely, imagine a being which shares none of our material properties but are, nevertheless, conscious, referred to as a Ghost. If either of these can exist then a conscious state, such as pain, cannot be identical to a brain state.
For physicalism to hold then Zombies and Ghosts cannot exist despite there being an intuition that there is no reason why they shouldn’t. For instance, we cannot tell for sure whether any of our fellow humans share our conscious states or not even if the behave in every way as expected. Such scenarios have been depicted in a number of science fiction stories, eg [i][ii]. Moreover, it is accepted that all of human behaviour can be modelled without any reference to phenomenal consciousness. We only know it exists because of our own experience.
Although Papineau [] argues that Zombies are impossible, it appears that he only shows that they are not necessary as a consequence of it being possible to conceive of them. Moreover, later in his book, he refers to experiments with non-verbal but intelligent monkeys and says this:
“You prompt the monkeys in various ways, you get them to perform various tasks, and you check what is going on in their brains at the same time. … However it won’t tell us anything at all about monkeys’ phenomenal consciousness. Without any first person reports to go on, it is perfectly consistent with such investigations that monkeys have no phenomenal conscious at all, or a full phenomenal life just like ours or anything in between” [ page 182]
So the monkeys could be Zombies and we would have no way of knowing. Similarly an alien species who could not understand our language would come to the same conclusion about humans.
iSheeri S Tepper, “The True Game”,
iiTad Williams, “Otherland”