In a slight departure I am going to present something of a traditional philosophical problem. That is, I will present the problem as it typically appears in philosophical circles: I may not end up responding to it in such a traditional manner.
The problem in question is called that of Other Minds. Philosophers are oddly prejudiced in that only a scant few diehard skeptics would spend much time doubting the existence or status of their own minds, whereas plenty are in some form at least perfectly happy to doubt the status of other minds. In general one’s own experience, sensations, feelings etc. are immediate and unquestioned. We know our experience of the world comes with attendant mental states. But, unable to so directly access the experience of others, an apparent gap is cited.
There is even an argument. And within the argument is betrayed the underlying agenda: that this is not truly about skepticism regarding the minds of others, but rather about the commitment of certain philosophers to the mind not being physical. Anyway, the argument can be presented as such:
Physicalism (the idea that all that exists is physical) entails that everything, including mental states, is physical.
If physicalism is true then any possible world physically identical to the actual world contains all and only everything, including mental states, that exists in the actual world.
However it is possible to conceive of a possible world physically identical to our own devoid of mental states.
Therefore physicalism is false (and, further, mental states are distinct from physical states).
Responding traditionally, in a sense, I will say this: try not to trust philosophers using ‘possible world’ as a term. Intended to be shorthand for simply imagining a counterfactual state of affairs it gets tangled up with confusion and baggage and metaphysical detritus. All it should serve to do here is to say we can imagine a physically identical world (and, let’s face it, that means thinking of this world) without mental states (that is, where all known behaviour is still observed, it is just without attendant consciousness). But can we? If we can, or believe we can, we have just begged the question (in the proper sense), since you have to already understand mental states to be something separate from the physical to draw the conclusion.
If one genuinely believes that mental states are simply a function of physical entities and processes then imagining a physically identical world will also contain those mental states. Further, if one admits to not believing mental states to being physical then the whole thought experiment is revealed as pointless.
Worse: there is something of the worst kind of reasoning to be found in philosophy here. Almost no philosopher positing this picture of mental states not being coextensive with physical states is actually claiming such philosophical ‘zombies’ are real, or even physically possible. The entire position is based upon establishing a logical possibility – borne out the apparent ability to imagine a physically identical world that has no mental states – which in turn allows a real-world conclusion to be drawn. That if we can imagine such a world, then in some sense we can conclude a fundamental separation between the mental and physical.
I, for one, am not convinced. I do not want to base my real-world understanding on the feeble imaginative power of a conflicted interest dualist (one who believes mental and physical are fundamentally different substances). It may be bad form in philosophical circles to imply any particular argument or thought experiment is anything other than the cold application of reason, but one has to wonder exactly why someone would posit this argument. Despite the relentless and manifest march of our understanding of all real elements of the world being physical there persists a huge section of philosophers hell-bent on preserving a wedge between the world and their precious consciousness.
And it is a pathetic prejudice. Whether they wish to retain notions of soul, of post-physical persistence of self, or whether they simply feel a reduction of the mental to the physical is a reduction in a more moral or evaluative sense, doesn’t really matter. It is thin, obvious, special pleading wrapped up in the direst of philosophical traditions: logical validity.