Suppose that phenomenal properties, such as the intrinsic feel of pain, are not physical properties in the standard sense. It’s a fair bet that science will be able to identify physical causes for all the effects of experience. So how can phenomenal properties have any effect on us? How does the feel of our experiences make a difference to us?
Panpsychists have a neat answer to this. They say that the phenomenal properties of experience are the intrinsic natures of the physical states that play the functional roles of the relevant experiences, including causing their characteristic effects. So (to use a hoary example) if the firing of C-fibres in the brain is the physical state that plays the functional role of pain, then the feel of pain is the intrinsic nature of C-fibre firing. Assuming this intrinsic nature does at least some of the causal work in producing pain effects, then it follows that the feel of pain is a cause of pain behaviour.
This view is, in effect, a radical form of type identity theory or central-state materialism. It says that pain is type identical, not with the brain state that plays the pain role, but with the essential nature of that brain state. We might call the view essential-state materialism.
And like type identity theory, the view faces an objection from multiple realization. The same functional role could be played by different physical states. This may actually happen in other creatures — octopuses, perhaps — and we can easily imagine it happening in us. The brain is highly plastic, and existing structures can be recruited to play new roles in response to damage. In the future, we may even be able to replace damaged brain structures with artificial ones that play the same role.
This poses an obvious problem for the essential-state view. If pain is the essential nature of C-fibre firing, then creatures who lack C-fibres cannot feel pain, even if they have functionally identical states and respond exactly as if they do feel pain. This is a counterintuitive, and possibly cruel, view. How do panpsychists respond?
It is implausible to say that all the physical states that could play the pain role have the same intrinsic nature. (And even if it weren’t, it wouldn’t solve the problem, since those states could also play other functional roles, with the result that pain could cause behaviour quite unrelated to pain, which is equally counterintuitive.) And of course panpsychists can’t say that the phenomenal nature of a physical state changes with the functional role it plays, since that would mean that a physical state’s phenomenal nature is not intrinsic to it.
I’d be grateful for any thoughts on this problem or for references to discussions of it in the panpsychist literature.