Tuesday, March 10, 2009

Quantum Computing and Mind Simulation

At the end of a recent discussion (in the comments to this post), Allen and I started to discuss whether quantum computing theory had something to say about the potential for simulating the human mind on a computer. I then read a couple of review articles on quantum computing he referenced. Before getting to what I learned (below), I wanted to explain my prior philosophical view on the simulation question.

The Russellian Stance, Functionalism, and Simulation

I endorse a form of the Russellian approach to solving the hard problem of consciousness. Russell described the world as a causal network of events, and he noted that physical theory only describes the extrinsic or dispositional nature of these events. These events also have an intrinsic nature which ultimately grounds the qualitative and experiential character of consciousness. I think quantum mechanics provides support for this view: quantum measurements seem to fit perfectly into this Russellian picture as the base-level events which ultimately underpin both the physical and experiential facts.

Functionalism is the thesis that the mind can be described as an abstract causal system. As a practical matter, the functionalist’s description is taken at a coarse-grained level – i.e. there is some minimal scale below which the actual physical details of the brain/body system are assumed to be irrelevant to its function. It follows that such a functional model could be realized in any number of physical ways, including via computer. Computationalism is the variety of functionalism which pursues the computer modeling approach.

Now, I think the Russellian stance on functionalism and the potential for simulation is nuanced. On the one hand, functionalism is seen as misguided because it only considers extrinsic causal structure. On the other hand, unlike an old fashioned dualist, the Russellian shouldn’t rule out the possibility that the mind could be simulated. The mind, after all, is a product of a natural system – we don’t need extra immaterial stuff to explain it. Perhaps a simulation can get the functional structure right and the correct intrinsic experiential character will come along “for free.”

The problems come with the coarse-graining. In every functionalist account I’ve seen, this takes place at a scale where quantum mechanics is assumed to be safely irrelevant. But every process in the body ultimately is grounded in molecular, atomic and sub-atomic activity which must be described quantum mechanically. So, a coarse-grained, approximated simulation of the brain/body’s causal structure on some physical device would likely miss crucial details which lie at the quantum level (details I think simultaneously crucial to both extrinsic function and conscious experience)

How would a functionalist respond to the quantum question? First, many believe distinctive quantum phenomena effectively “wash out” in a macroscopic system like the human brain/body. This belief is often based on the presumed impact of environmental decoherence. I’m not going to pursue this issue in this post (I discuss this in some of my posts on quantum biology). Another response to the quantum question is an appeal to a commonly believed thesis that any physical system (including a quantum system) can be modeled by a classical computer -- so the traditional functionalist/computationalist approach wouldn’t be missing anything distinctive anyway. This is the view that I wanted to explore by reading up on quantum computing theory.

[Please note the discussion that follows may suffer even more than usual from by my ignorance of the subject matter.]

Simulation and the Church-Turing thesis

So, is it true that any physical system, including a quantum system, can be simulated by a classical computer? Well, this idea has been defended by appeals to versions of the Church-Turing thesis. The original Church-Turing thesis states (in a formulation from this article) that any effectively calculable function can be computed using a Turing machine. (For a description of a Turing machine, see here.) Now it seems that what this thesis really meant can probably only be appreciated by studying its original logical/mathematical context. In his SEP article on the C-T thesis, Jack Copeland first traces the development of the ideas associated with the thesis in the pioneering work on calculation and computing by Turing, Church and others beginning in the 1930’s. Then, Copeland spends much of the rest of the article objecting to how the thesis has been misunderstood and misused by philosophers and computing theorists. The C-T thesis did not purport to say that all physical systems, or even all machines regardless of architecture, could be simulated by a Turing machine. It certainly did not prove anything of the sort. (This discussion reminded me of similar debates over the philosophical applicability of Gödel’s incompleteness theorems beyond their original context – see old posts here and here.)

Nevertheless, as long as one is careful not to inappropriately invoke the authority of the original C-T thesis, one can explore more expansive versions and try to evaluate their validity.

Physical Versions of the C-T thesis

In her review article on Quantum Computing, Dorit Aharonov presents two versions of the thesis. First, (p.3), she presents a simple “physical” version: “A Turing machine can compute any function computable by a reasonable physical device.” She says this is something which cannot be proven, but that no known counterexamples exist. In particular, quantum computers are not believed capable of computing functions non-computable by a Turing machine.

She quickly then notes that: “However, in the theory of computation, we are interested not only in the question of which functions can be computed, but mainly in the cost of computing these functions.” [Emphasis original] The way this is evaluated is by noting whether the computational resources needed rise as a polynomial or an exponential function of input size. It is the former which form the set of tractable computations.

After also discussing the superior efficiency of a probabilistic version of the Turing machine, she presents another thesis to consider (p.4): “The modern Church thesis: A probabilistic Turing machine can simulate any reasonable physical device in polynomial cost.” We have a great deal of evidence, though not proof, that this thesis is contradicted by quantum computers.

The Advantages of Quantum Algorithms

Aharonov explains first, that quantum computers can simulate classical computers, at little loss in efficiency. On the flip side, it appears classical computers can simulate quantum computers but only at exponential cost. What we really want, though, is a positive demonstration of how far quantum computers can outperform their classical counterparts.

The a priori expectation might be that the ability to manipulate qubits, which can be in a superposition of states as opposed to just to two states, would lead to great increases in computing power. Because of the necessity for conducting a measurement to extract results (collapsing superpositions), however, the power of this idea is muted. Other, more subtle sources of limitations on quantum computing are discussed later in the paper.

Despite this, however, many investigations into quantum computing over the years have found quantum algorithms which improve efficiency. One of these algorithms, Shor’s, gives a polynomial algorithm for factoring integers where all known classical algorithms have exponential cost, thus crossing the crucial boundary. It must be pointed out that as of yet there is no proof that a classical polynomial algorithm for factoring is impossible.

Conclusions

Most of what I’ve discussed in Aharonov’s article above comes from the introduction. In the ensuing 60 pages she goes into more detail about the nature of computers and computing, various models for quantum computing algorithms, the issues of noise correction and fault tolerance, and some of her own ideas of what quantum computing theory says about the boundary between quantum and classical regimes in physics.

On the key question of what quantum computers can do better than classical ones, one is left with the impression that the question is much more subtle than might first be imagined. We have some exciting theoretical results, but perhaps fewer than might have been anticipated on a naïve expectation. At the same time, it seems we’re still in the early phase of growth in our knowledge of the field. A lot of interesting work and new developments lie ahead. (The engineering efforts underway toward building quantum computers and the challenges they face is another interesting topic).

Let me return to the question about what all this might mean for the philosophy of mind, assuming (as I do) that the quantum level grounding of biology contributes meaningfully to the mind’s function. I think a modest conclusion is called for. The fact that a classical computer can simulate a quantum computer only at an exponential cost suggests that the project of simulating a human mind is impractical, though not blocked in principle. This conclusion is broadly consistent with my philosophical stance regarding the simulation project.

[P.S: after drafting this I recalled there was a good debate (thanks to Tanasije and Mike) on the simulation topic in the comments to this May 2008 post on Russellian theory. My memory is awful.]

19 comments:

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M*P*Lockwood said...

“A Turing machine can compute any function computable by a reasonable physical device.” She says this is something which cannot be proven, but that no known counterexamples exist."

I thought that was exactly what Turing himself proved mathematically - that anything computable at all is computable by a Universal Turing Machine. This article seems to back me up. http://en.wikipedia.org/wiki/Universal_Turing_machine

But regarding the content of yr post: I can believe that a quantum computer might make a simulation of a human mind run faster, but even if quantum computing is very helpful, that doesn't mean that a Turing computer couldn't do the job in principle, just that the simulated mind might think very slowly.

I am quite confident that we will find that the most efficient method for constructing a computer which simulates the mind is to build it out of neurons - after all, evolution has been working on this problem for millions of years.

Steve said...

Thanks very much for commenting.
On what Turing proved: the key word is "physical". As far as I understand these things, he was working in the abstract logical/mathematical realm, and the notion of proof doesn't carry over to the physical realm.

Allen said...

A very good post! A well written summary of a lot of interesting information.

So, after reading that, really it seems to me that your theory of consciousness boils down to just "intrinsic properties cause consciousness".

If a quantum mechanical system's "extrinsic" properties and behaviors (including entanglement and tunneling and all the rest) can be simulated by a classical computer (even though perhaps only at exponential cost), then quantum mechanics wouldn't seem to contribute anything special to conscious subjective experience.

It would seem that all that quantum mechanical processes could be said to contribute are raw processing power, above and beyond what is efficiently available from classical systems (maybe).

So, all of the "specialness" that explains consciousness would have to come from the intrinsic properties, wouldn't it?

Though, as soon as those intrinsic properties exert an observable influence into the universe, then they wouldn't be "intrinsic" properties anymore...they'd be extrinsic. And if they exert their influence in a regular law-like way...then they'd basically just be observable forces of nature, like gravity, electromagnetism, and the strong and weak forces.

For these properties to stay intrinsic, they basically have to be carried along by the observable extrinsic forces, right? If they do anything to influence the course of events at the physical level, then we're back to them being extrinsic and thus subject to explanation by physics.

But once the right set of intrinsic properties are carried along by their extrinsic counterparts into the right physical configuration, then consciousness results? Does consciousness have "emergent" causal powers? Do we get into issues of downwards causation at this point? Are you saying that the "random" aspect of results from quantum measurements aren't really random? That they are somehow biased in favor of outcomes that produce consciousness, or maybe that the randomness is a side-effect of consciousness?

Also, from your post:

>> Russell described the world as a causal network of events

Do these events directly cause our perceptions? OR do these events cause the material universe which then in turn causes our perceptions?

Do we need intrinsic properties to explain anything other than consciousness? Or is that really their only purpose in your theory?

Steve said...

Hi Allen. Here’s an attempt at some responses.

>>So, after reading that, really it seems to me that your theory of consciousness boils down to just "intrinsic properties cause consciousness".

Cause might not be the right word – that’ll get us into trouble later -- I’d like to say they comprise consciousness.

>>If a quantum mechanical system's "extrinsic" properties and behaviors (including entanglement and tunneling and all the rest) can be simulated by a classical computer (even though perhaps only at exponential cost), then quantum mechanics wouldn't seem to contribute anything special to conscious subjective experience.

>>It would seem that all that quantum mechanical processes could be said to contribute are raw processing power, above and beyond what is efficiently available from classical systems (maybe).

When you have your computing hat on, I think that is right, but the quantum measurement event is at the root of the intrinsic/qualitative/experiential aspect of nature.

>>So, all of the "specialness" that explains consciousness would have to come from the intrinsic properties, wouldn't it?

Yes – the special part being the subjective and qualitiative character (the sources of the hard problem).

>>For these properties to stay intrinsic, they basically have to be carried along by the observable extrinsic forces, right? If they do anything to influence the course of events at the physical level, then we're back to them being extrinsic and thus subject to explanation by physics.

Philosophers who have theories along these lines usually have the extrinsic properties being carried by the intrinsic ones, rather than vice versa. But for this discussion I don’t think that matters much. I tend to think they just go together, and we just get acquainted with them depending on our point of view – as a participant in an interaction, or as an indirect observer or evaluator.

>>Does consciousness have "emergent" causal powers? Do we get into issues of downwards causation at this point?

I’d say no and no; intrinsic properties, and hence experiences, are part of causation at the base-event level.

>>Are you saying that the "random" aspect of results from quantum measurements aren't really random? That they are somehow biased in favor of outcomes that produce consciousness, or maybe that the randomness is a side-effect of consciousness?

The probabilistic aspect of QM is fundamental – I didn’t’ think anything here speaks to that subject. An event collapses the wave function.

>>Do these events directly cause our perceptions? OR do these events cause the material universe which then in turn causes our perceptions?

Events comprise both the “material” universe (when considered from the third person perspective) and conscious experience (when you have first person participation).

>>Do we need intrinsic properties to explain anything other than consciousness? Or is that really their only purpose in your theory?

Intrinsic properties are part of everything. It would seem we only need them to explain consciousness, if everyone seems happy with purely functional descriptions of everything else. However, Russell and some others would say that they are indeed needed to put the concrete “meat” onto the mathematical “bones” of our theories of nature generally; it’s a mistake to take the mathematical modeling of extrinsic relations as telling the whole story about anything.

Allen said...

>>When you have your computing hat on, I think that is right, but the quantum measurement event is at the root of the intrinsic/qualitative/experiential aspect of nature.

BUT, a quantum measurement event, without intrinsic properties, buys you nothing, right?

And, WITH intrinsic properties, it's possible to imagine a "classical" universe, even a billiard-ball universe, where consciousness is possible, isn't it?

So it seems to me that quantum wave function collapse just provides a conveniently "mysterious" hook to hang your intrinsic properties on.


>> However, Russell and some others would say that they are indeed needed to put the concrete “meat” onto the mathematical “bones” of our theories of nature generally

I think this is another example of just pushing the question one step further down the ladder. So we aren't satisfied with what we see in the universe as a basis for our explanation of consciousness. So let's infer some UNSEEN stuff to explain it. But this in turn just raises questions about the unseen stuff. What are these experiential events, ultimately? Where do they come from? Why do they have the intrinsic/extrinsic properties that they have? Why does it work the way that it works? Could they work differently? Are there other types of events/event networks that give rise to other types of universes? If not, why not?


>> it’s a mistake to take the mathematical modeling of extrinsic relations as telling the whole story about anything

What I like about more abstract platonic theories is that mathematical truths are necessary truths. No matter what universe you find yourself in, they will hold. They don't even depend on physical instantiation, they are true regardless of whether anyone ever discovers them.

It seems pretty reasonable to me to say that what the brain ultimately does is process information. Consciousness is then either a side-effect of either the information processing, or of the information itself. Either way it exists platonically, independent of physical universes of any kind.

Revling said...

Wow, Steve, great discussion here. So, um, where's the rss feed?

Steve said...

Hi Revling. I put some URL's on the side bar for feeds -- I'd appreciate it if you let me know if they work for you.

Steve said...

>>BUT, a quantum measurement event, without intrinsic properties, buys you nothing, right?

>>And, WITH intrinsic properties, it's possible to imagine a "classical" universe, even a billiard-ball universe, where consciousness is possible, isn't it?

>>So it seems to me that quantum wave function collapse just provides a conveniently "mysterious" hook to hang your intrinsic properties on.

I think that is good criticism – Russell didn’t necessarily have QM in mind when he advocated his view in 1927. But I think the fact that quantum measurement events irreducibly take into account the point of view of the measuring system (this is the subject of my next post) shows that quantum mechanics has a more natural home for the intrinsic properties.


>>What are these experiential events, ultimately? Where do they come from? Why do they have the intrinsic/extrinsic properties that they have? Why does it work the way that it works? Could they work differently? Are there other types of events/event networks that give rise to other types of universes? If not, why not?

I think metaphysical explanation has to start somewhere – with a fundamental entity or entities. These events are my candidate.


>>What I like about more abstract platonic theories is that mathematical truths are necessary truths. No matter what universe you find yourself in, they will hold. They don't even depend on physical instantiation, they are true regardless of whether anyone ever discovers them.

>>It seems pretty reasonable to me to say that what the brain ultimately does is process information. Consciousness is then either a side-effect of either the information processing, or of the information itself. Either way it exists platonically, independent of physical universes of any kind.

Consciousness is the process of instantiation.

I speculate that uninstantiated information does exist platonically – from our point of view. Somewhere out there in the multiverse from other points of view this other information is instantiated and the events of our world are platonic entities.

dnn8350 said...
This comment has been removed by the author.
dnn8350 said...

I know this comment comes a bit late in the day, but I hope it's still relevant. Your contention is that, since the "feel" of consciousness seems not to be explicable as an "extrinsic" aspect of function, it may instead derive from some additional "intrinsic" quality. If so, how are we to explain our propensity to refer to such "functionally opaque" qualities, if the referring itself (i.e. the "access" account of consciousness) is deemed to be entirely functionally explicable? As an analogy, computer software can have no independent knowledge of, and hence no motivation to refer to, whatever underlying hardware may be responsible for its execution, because the "intrinsic" qualities of such hardware are hidden from the software "point of view".

Steve said...

Hi. That's a good question. But I think the functional (extrinsic) and qualitative (intrinsic) are unified at the most fundamental physical level. I'm not arguing for dualism, but a monism. So maybe it's not mysterious that the functional/dispositional aspects of consciousness can include the impetus to refer to the qualitative aspect.

See, I don't like the software/hardware analogy -- since it seems to assume (as functionalist/computationalist models do) that the software can be implemented on a variety of hardware platforms. But that's what I'm denying. Consciousness is the way it is because of its instantiation at the quantum level where "software" and "hardware" (the qualitative and dispositional aspects of things) are inseparable/unified.

dnn8350 said...

I know you like Greg Rosenberg's ideas, but as I recall at the very end of his book he still had difficulty with this aspect - i.e. even with receptive properties and carrier structure in place, we still want the effective account to be functionally "complete" (at least in principle). But if this is to be so, it would seem to leave no purchase for additional "impetus" from the qualitative aspects. Hence, as I think he admits, our ability to refer to them still seems puzzling. His proposal, if I've understood him, seems to be a sort of overdetermination of reference, in that a "functional" or effective analogue of qualitative reference is somehow simultaneously "realised" by the qualitative substructure. Actually, this seems not implausible to me. To put it crudely, if one omits the circular thinking and question begging that is customary in this area, the purely effective or extrinsic description of the physical world can plausibly be reduced (ontologically, that is) to the level of particle interactions. Unless one is motivated to posit strong causal emergence at some higher level (i.e. "downward causation"), all composite structure (i.e. our macroscopic environment) would consequently seem, in some fundamental sense, to be more properly an aspect of epistemology - i.e. an a posteriori interpretation of a micro-physical "zombie" machine that seemingly runs perfectly happily on its own. If this be so, then the "realisation" of the qualitative counterpart of effective properties would in fact be constitutive of our entire experiential world of composite objects and events. This might be a sufficient justification for the removal of any accusation of "epiphenomenalism"!

Steve said...

That's interesting, although I'm not completely sure I understand. I'm heading out for a trip, and I'll follow up a bit later. Thanks. - Steve

Steve said...

Ok , the qualitative aspect of things seems necessary for consciousness and real causality, but still doesn’t seem needed for a complete functional description of humans. So the problem is why do we have dispositions to make reference to the qualitative side? (Let’s assume this is indeed a problem, although I have some feeling that perhaps it shouldn’t be surprising if the qualitative/dispositional divide is only in our descriptions of nature, not in nature itself.)

You describe Rosenberg’s idea (and I haven’t reviewed his book again on this yet) as:

His proposal, if I've understood him, seems to be a sort of overdetermination of reference, in that a "functional" or effective analogue of qualitative reference is somehow simultaneously "realised" by the qualitative substructure.

You then say:

…the purely effective or extrinsic description of the physical world can plausibly be reduced (ontologically, that is) to the level of particle interactions…

and

all composite structure (i.e. our macroscopic environment) would consequently seem, in some fundamental sense, to be more properly an aspect of epistemology - i.e. an a posteriori interpretation of a micro-physical "zombie" machine that seemingly runs perfectly happily on its own. If this be so, then the "realisation" of the qualitative counterpart of effective properties would in fact be constitutive of our entire experiential world of composite objects and events. This might be a sufficient justification for the removal of any accusation of "epiphenomenalism"!

So, are you saying that the fact that our experiential field includes “lumpy” macro objects and events (rather than a chaos of particles) is evidence of the work the qualitative side is doing?

dnn8350 said...

The part of Rosenberg's book I had in mind is in "The Consciousness Hypothesis" chapter under "the knowledge paradox" (page 258, Oxford hard-back edition). He says "The knowledge paradox stems from our knowing we are conscious even though experiencing seems not to be causally responsible for our brain states. On the carrier view, one might have a nagging feeling that causal responsibility hangs only off the effective side of things." This is indeed my point of departure.

He goes on to describe a direct knowledge by acquaintance - a "knowing what" - which (crucially for his argument) is not "truth evaluable": e.g. knowing what it is like to hear a scream, to smell a baby, etc. "The epistemic puzzle for consciousness", he says, "does not concern how we may have knowledge in the "knowing what...." sense" because "this knowledge is knowledge of the basic causal nature of the particular we are." I concur with this if only because something of the sort seems necessary to block an infinite regress of 'homunculi'. But, he says, "the problem is to explain how this "knowing what......." justifies instances of propositional knowledge expressed by "knowing that....." clauses. How does the intimacy of acquaintance license the uttering of sentences?" In my view, these comments are the pivot of the entire book because, lacking a plausible reconciliation of these types of knowing and a link to the behaviour thus engendered, his whole project founders.

He goes on to sketch how "knowing that can emerge from knowing what with the help of knowing how." There isn't space to recapitulate his entire argument here, but he concludes "Even though consciousness is not physical, its activity underlies our physical nature as a carrier of our nomic content. Our physical states, although not causally interacting with our conscious states, track and therefore represent those states.........phenomenal properties are used to represent themselves."

The section on the knowledge paradox is dense and rather subtle. He concludes by conceding that the "effective" side of things should indeed be understood as being entirely responsible for our judgements and utterances about conscious experiences. This is the point, of course, at which Dennett and the functionalist/computationalist school rest their case, and indeed any substantially divergent conclusion would be problematical, to say the least, for current theories of micro-physical reality. But Rosenberg of course doesn't rest here; he argues that the "effective narrative" is not the whole story: the complex of effective action, that (construed somehow) must comprise the objects and events of our experience and the justification of our behaviour, systematically "tracks" its phenomenal counterpart.

Having run out of space, I'll say more about this in the following comment.

dnn8350 said...

Rosenberg's ideas about "tracking" really got me thinking again about some speculations of my own. The purely "effective" story is what Dennett et al would have us believe is the whole schmear ("who you gonna believe", as Groucho Marx once asked an incredulous Chico "me or your own eyes?"). It's also, in effect, the story of Chalmers' zombie - the doppelganger that is "effectively" identical but phenomenally lacking. Finally, it's the narrative of physical science tout court. But is all this to presume too much? Are these purely effective versions of events in fact helping themselves to levels of "reality" to which they are not, viewed strictly in their own terms, entitled? After all, if we eliminate any idea of "strong causal emergence" (which is at least the implicit assumption of micro-physical theory - i.e. quantum mechanics is a complete theory of both the micro-and macro-scopic) what do we have but particles and their interactions? It's no use appealing for anything further to the composite reality of our experience, or pointing out that QM isn't much help in understanding or predicting anything beyond two or perhaps three-particle interactions, because this is all after the fact - i.e. it is begging the question by citing the evidence of experience, the very matter under question. After all, according to QM, there is a sort of micro-physical machine that does all the work at the fundamental level, and this level is all that Occam would appear to need to account for anything and everything that exists and occurs (OK there's the small question of gravity, but you get my drift).

If god is contemplating all of this from some peaceful spot outside of everything, presumably she may be perfectly cognizant of the details of how some particle at the tip of what I am pleased to call my nose arrived at that point in spacetime on the basis of what was initiated at the big bang, without invoking any of the complexly derived entities and events that notionally might constitute "me" or my "macroscopic environment". The fundamental point in Chalmers' zombie argument gets missed, I think, even by him. When we imagine the zombie we should invoke, not a human-like facsimile uttering the same statements and entertaining the same thoughts and beliefs, but rather the bare micro-physical effective substructure prior to any such interpretation. Indeed, looked at in this way, the zombie is US. And god might well have arranged matters this way ("just the facts, maam"). But she didn't. And the evidence to the contrary is precisely our knowledge by acquaintance of an integrated environment of composite objects and events that transcends (but doesn't contradict) the reductive narrative. This knowledge has the aspect of an INTERPRETATION projected on (or integrated over) the microphysical story. Without that knowledge by acquaintance, god might still entertain herself by imagining our "zombie" or "virtual" selves existing and functioning in terms of that "interpretative environment", but we ourselves would have no purchase on any reality beyond our particulate existence.

Steve said...

That's a very interesting idea. It's good to hear from someone who's been doing some serious thinking on this.

dnn8350 said...

I'm glad you think so Steve. Having done a fair amount of googling, the only similar view I've managed to dredge up was expressed by Jaron Lanier during an online conversation, when he speculated that "without consciousness, perhaps there's just quarks". I usually find it hard going to get people to appreciate why there might be a problem with our usual assumptions about this. Having posted yesterday's comments, I happened to be having a conversation in the pub with some philosophically-inclined friends, and tried the same exposition out on them - they had absolutely no clue what I was talking about. I pointed out that I wasn't peddling some knock-down mind-body theory, just trying to to see where our assumptions about "physical reality" lead if you're actually prepared to take them seriously. They just kept frowning and saying "what's the problem - yes, of course there are particles, but they're arranged into macroscopic objects and that's what we experience". No matter how often I insisted that this "conclusion" was in fact an unavoidable but irremediably a posteriori assumption founded in an implicit overdetermination of the elements of physical reality, they still couldn't grasp my point. Realism - "naive" or otherwise - had them in its relentless grip. "Semantics!" they said. I gave up.

This "assumption" is of course our common situation, and I certainly don't have any definitive solution of how it is to be reconciled with a micro-physical "completist" account. Maybe Rosenberg's approach will turn out to lie in the right direction - i.e. a self-consistent macroscopic history (his "natural individuals") condensing from the microscopic flux through the influence of hidden layers of casual structure. But my own suspicions, for what they're worth, don't really lie in the direction of hidden properties of matter; I actually find the functional/computational approach rather persuasive (as indeed does Rosenberg) but incomplete (as again does he). My argument against Dennett-style eliminativism about consciousness is that, rigorously applied, it denies not merely phenomenal, but macroscopic, reality and thus radically fails to save the appearances in its bare form. Somehow the localisation of a "virtual" first person environment from a global third-person ensemble of maximal fragmentation comes to be "realised" in this sui-generis form. Somehow two "views of reality" - global and local, whole and part - intersect, neither being dispensable without the loss of some crucial element of truth. There is indeed a suggestive similarity in this to the "hardware"/"software" relationship, but perhaps one in which the "hardware" isn't what we take it to be and the "software" is no mere metaphor but a fundamental level of reality.