First, [Brown] cites my 1990 discussion piece “How Cartesian dualism might have been true”, in which I argued that creatures who live in simulated environments with separated simulated cognitive processes would endorse Cartesian dualism. The cognitive processes that drive their behavior would be entirely distinct from the processes that govern their environment, and an investigation of the latter would reveal no sign of the former: they will not find brains inside their heads driving their behavior, for example. Brown notes that the same could apply even if the creatures are zombies, so this sort of dualism does not essentially involve consciousness. I think this is right: we might call it process dualism, because it is a dualism of two distinct sorts of processes. If the cognitive processes essentially involve consciousness, then we have something akin to traditional Cartesian dualism; if not, then we have a different sort of interactive dualism.

Looking back on this now I think that I can say that part of the idea I had was that what Dave here calls ‘process dualism’ is really what lies behind the conceivability of zombies. Instead of testing whether (one thinks that) dualism or physicalism is true about consciousness the two-dimensional argument against materialism is really testing whether one thinks that consciousness is  grounded in biological or functional/computational properties. This debate is distinct and orthogonal to the debate about physicalism/dualism.

In the next part of the response Dave addresses my attempted extension of this point to try to reconcile the conceivability of zombies with what I called ‘biologism’. Biologism was supposed to be a word to distinguish the debate between the physicalist and the dualist from the debate between the biologically-oriented views of the mind as against the computationally oriented views. At the time I thought this term was coined by me and it was supposed to be an umbrella term that would have biological materialism as a particular variant. I should note before going on that it was only after the paper was published that I became aware that this term has a history and is associated with certain views about ‘the use of biological explanations in the analysis of social situations‘. This is not what I intended and had I known that beforehand I would have tried to coin a different term.

The point was to try to emphasize that this debate was supposed to be distinct from the debate about physicalism and that one could endorse this kind of view even if one rejected biological materialism. The family of views I was interested in defending can be summed up as holding that consciousness is ultimately grounded in or caused by some biological property of the brain and that a simulation of the brain would lack that property. This is compatible with materialism (=identity theory) but also dualism. One could be a dualist and yet hold that only biological agents could have the required relation to the non-physical mind. Indeed I would say that in my experience this is the view of the vast majority of those who accept dualism (by which I mostly mean my students). Having said that it is true that in my own thinking I lean towards physicalism (though as a side-side note I do not think that physicalism is true, only that we have no good reason to reject it) and it is certainly true that in the paper I say that this can be used to make the relevant claim about biological materialism.

At any rate, here is what Dave says about my argument.

Brown goes on to argue that simulated worlds show how one can reconcile biological materialism with the conceivability and possibility of zombies. If biological materialism is true, a perfect simulation of a biological conscious being will not be conscious. But if it is a perfect simulation in a world that perfectly simulates our physics, it will be a physical duplicate of the original. So it will be a physical duplicate without consciousness: a zombie.

I think Brown’s argument goes wrong at the second step. A perfect simulation of a physical system is not a physical duplicate of that system. A perfect simulation of a brain on a computer is not made of neurons, for example; it is made of silicon. So the zombie in question is a merely functional duplicate of a conscious being, not a physical duplicate. And of course biological materialism is quite consistent with functional duplicates.

It is true that from the point of view of beings in the simulation, the simulated being will seem to have the same physical structure that the original being seems to us to have in our world. But this does not entail that it is a physical duplicate, any more than the watery stuff on Twin Earth that looks like water really is water. (See note 7 in “The Matrix as metaphysics” for more here.) To put matters technically (nonphilosophers can skip!), if P is a physical specification of the original being in our world, the simulated being may satisfy the primary intension of P (relative to an inhabitant of the simulated world), but it will not satisfy the secondary intension of P. For zombies to be possible in the sense relevant to materialism, a being satisfying the secondary intension of P is required. At best, we can say that zombies are (primarily) conceivable and (primarily) possible— but this possibility mere reflects the (secondary) possibility of a microfunctional duplicate of a conscious being without consciousness, and not a full physical duplicate. In effect, on a biological view the intrinsic basis of the microphysical functions will make a difference to consciousness. To that extent the view might be seen as a variant of what is sometimes known as Russellian monism, on which the intrinsic nature of physical processes is what is key to consciousness (though unlike other versions of Russellian monism, this version need not be committed to an a priori entailment from the underlying processes to consciousness).

I have to say that I am sympathetic with Dave in the way he diagnoses the flaw in the argument in the paper. It is a mistake to think of the simulated world, with its simulated creatures, as being a physical duplicate of our world in the right way; especially if this simulation is taking place in the original non-simulated world. If the biological view is correct then it is just a functional duplicate, true a microfunctional duplicate, but not a physical duplicate.

While I think this is right I also think the issues are complicated. For example take the typical Russellian pan(proto)psychism that is currently being explored by Chalmers and others. This view is touted as being compatible with the conceivability of zombies because we can conceive of a duplicate of our physics as long as we mean the structural, non-intrinsic properties. Since physics, on this view, describes only these structural features we can count the zombie world as having our physics in the narrow sense. The issues here are complex but this looks superficially just like the situation described in my paper. The simulated world captures all of the structural features of physics but leaves out whatever biological properties are necessary and in this sense the reasoning of the paper holds up.

This is why I think the comparison with Russellian monism invoked by Dave is helpful. In fact when I pitched my commentary to Dave I included this comparison with Russellian monism but it did not get developed in the paper. At any rate, I think what it helps us to see is the many ways in which we can *almost* conceive of zombies. This is a point that I have made going back to some of my earliest writings about zombies.  If the identity theory is true, or if some kind of biological view about consciousness is true, then there is some (as yet to be discovered) property/properties of biological neural states which necessitate/cause /just are the existence of phenomenal consciousness. Since we don’t know what this property is (yet) and since we don’t yet understand how it could necessitate/cause/etc phenomenal consciousness, we may fail to include it in our conceptualization of a ‘zombie world’. Or we may include it and fail to recognize that this entails a contradiction. I am sympathetic to both of these claims.

On the one hand, we can certainly conceive of a world very nearly physically just like ours. This world may have all/most of the same physical properties, excepting certain necessary biological properties, and as a result the creatures will behave in indistinguishable ways from us (given certain other assumptions). On the other hand we may conceive of the zombie twin as a biologically exact duplicate in which case we do not see that this is not actually a conceivable situation. If we knew the full biological story we would be, or at least could be, in a position to see that we had misdescribed the situation in just the same way as someone who did not know enough chemistry might think they could conceive of h2o failing to be water (in a world otherwise physically just like ours). This is what I take to be the essence of the Krpkean strategy. We allow that the thing in question is a metaphysical possibility but then argue that it is actually misdescribed in the original argument. While misdescribing it we think (mistakenly) we have conceived of a certain situation being true but really we have conceived of a slightly different situation being true and this one is compatible with physicalism.

Thus while I think the issues are complex and that I did not get them right in the paper I still think the paper is morally correct. To the extent that biological materialism resembles Russellian monism is the extent to which the zombie argument is irrelevant.