Harnad, S. (1993) Artificial Life: Synthetic Versus Virtual. Artificial Life III. Proceedings, Santa Fe Institute Studies in the Sciences of Complexity. Volume XVI.


Stevan Harnad
Cognition et Mouvement URA CNRS 1166
Universite d'Aix Marseille II
13388 Marseille cedex 13, France


ABSTRACT: Artificial life can take two forms: synthetic and virtual. In principle, the materials and properties of synthetic living systems could differ radically from those of natural living systems yet still resemble them enough to be really alive if they are grounded in the relevant causal interactions with the real world. Virtual (purely computational) "living" systems, in contrast, are just ungrounded symbol systems that are systematically interpretable as if they were alive; in reality they are no more alive than a virtual furnace is hot. Virtual systems are better viewed as "symbolic oracles" that can be used (interpreted) to predict and explain real systems, but not to instantiate them. The vitalistic overinterpretation of virtual life is related to the animistic overinterpretation of virtual minds and is probably based on an implicit (and possibly erroneous) intuition that living things have actual or potential mental lives.

The Animism in Vitalism

There is a close connection between the superstitious and even supernaturalist intuitions people have had about both life and mind. I think the belief in an immaterial or otherwise privileged "vital force" was, consciously or unconsciously, always parasitic on the mind/body problem, which is that conceptual difficulty we all have in integrating or equating the mental world of thoughts and feelings with the physical world of objects and events, a difficulty that has even made some of us believe in an immaterial soul or some other nonmaterial animistic principle. The nature of the parasitism was this: Whether we realized it or not, we were always imagining living things as having mental lives, either actually or potentially. So our attempts to account for life in purely physical terms indirectly inherited our difficulty in accounting for mind in purely physical terms.

It is for this reason that I believe all positive analogies between the biology of life and the biology of mind are bound to fail: We are invited by some (e.g., Churchland 1984, 1986, 1989) to learn a lesson from how wrong-headed it had been to believe that ordinary physics alone could not explain life -- which has now turned out to be largely a matter of protein chemistry -- and we are enjoined to apply that lesson to current attempts to explain the mind -- whether in terms of physics, biochemistry, or computation -- and to dismiss the conceptual dissatisfaction we continue to feel with such explanations on the grounds that we've been wrong in much the same way before.

But the facts of the matter are actually quite the opposite, I think: Our real mistake had been in inadvertently conflating the problem of life and the problem of mind, for in bracketing mind completely when we consider life, as we do now, we realize there is nothing left that is special about life relative to other physical phenomena -- at least nothing more special than whatever is special about biochemistry relative to other branches of chemistry. But in the case of mind itself, what are we to bracket?

The Mind/Body Problem: An Extra Fact

The disanalogy and the inappropriateness of extrapolating from life to mind can be made even clearer if we face the mind/body problem squarely: If we were given a true, complete description of the kind of physical system that has a mind, we could still continue to wonder forever (a) why the very same description would not be equally true if the system did not have a mind, but just looked and behaved exactly as if it had one and (b) how we could possibly know that every (or any) system that fit the description really had a mind (except by being the system in question). For there is a fact of the matter here -- we all know there really are mental states, and there really is something it's like to be a system that has them -- yet that fact is the very one that eludes such a true, complete description, and hence must be taken entirely on faith (a state of affairs that rightly arouses some skepticism in many of us; Nagel 1974, 1986; Harnad 1991).

Now let me point out the disanalogy between the special empirical and conceptual situation just described for the case of mind modelling and the parallel case of life-modelling, which involves instead a true, complete description of the kind of physical system that is alive; then, for good measure, this disanalogy will be extended to the third parallel case, matter-modelling, this time involving a true complete description of any physical system at all (e.g., the world of elementary particles or even the universe as a whole): If we were given a true, complete description of the kind of physical system that is alive, could we still ask (a) why the very same description would not be equally true if the system were not alive, but just looked and behaved exactly as if it were alive? And could we go on to ask (b) how we could possibly know that every (or any) system that fit that description was really alive? I suggest that we could not raise these questions (apart from general worries about the truth or completeness of the description itself, which, it is important to note, is not what is at issue here) because in the case of life there just is no further fact -- like the fact of the existence of mental states -- that exists independently of the true complete physical description.

Consider the analogous case of physics: Could we ask whether, say, the planets really move according to relativistic mechanics, or merely look and behave exactly as if they did? I suggest that this kind of question is merely about normal scientific underdetermination -- the uncertainty there will always be about whether any empirical theory we have is indeed true and complete. But if we assume it as given (for the sake of argument) that a theory in physics is true and complete, then there is no longer any room left for any conceptual distinction between the "real" and "as-if" case, because there is no further fact of the matter that distinguishes them. The facts -- all objective, physical ones in the case of both physics and biology -- have been completely exhausted.

Not so in the case of mind, where a subjective fact, and a fact about subjectivity, still remains, and remains unaccounted for (Nagel 1974, 1986). For although the force of the premise that our description is true and complete logically excludes the possibility that any system that fits that description will fail to have a mind, the conceptual possibility is not only there, but amounts to a complete mystery as to why the description should be true and complete at all (even if it is); for there is certainly nothing in the description itself -- which is all objective and physical -- that either entails or even shows why it is probable that mental states should exist at all, let alone be accounted for by the description.

I suggest that it is this elusive extra (mental) fact of the matter that made people wrongly believe that special vital forces, rather than just physics, would be needed to account for life, But, modulo the problem of explaining mind (setting aside, for the time being, that mind too is a province of biology), the problem of explaining life (or matter) has no such extra fact to worry about. So there is no justification for being any more skeptical about present or future theories of life (or matter) than is called for by ordinary considerations of underdetermination, whose resolution is rightly relativized and relegated to the arena of rival theories fighting it out to see which will account for the most data, the most generally (and perhaps the most economically).

Searle's Chinese Room Argument Against Computationalism

How much worry does this extra fact warrant on its own turf, then, in empirical attempts to model the mind? None, I would be inclined to say (being a methodological epiphenomenalist who believes that only a fool argues about the unknowable; Harnad 1982a,b, 1991), but there does happen to be one prominent exception to the viability of the methodological strategy of bracketing the mind/body problem even in mind-modelling, and that exception also turns out to infect a form of life-modelling; so in some respects we are right back where we started, with an animistic consideration motivating a vitalistic one! Fortunately, this exception turns out to be a very circumscribed special case, easily resolved in principle. In practice, however, resolving it has turned out to be especially tricky, with most advocates of the impugned approach (computationalism) continuing to embrace it, despite counterevidence and counterarguments, for essentially hermeneutic (i.e., interpretational) reasons (Dietrich 1990; Dyer 1990; cf. Harnad 1990b, c): It seems that the conceptual appeal of systems that are systematically interpretable as if they were alive or had minds is so strong that it over-rides not only our natural skepticism (at least insofar as mind-modelling is concerned) but even the dictates of both common sense and ordinary empirical evidence -- at least so I will try to show.

The special case I am referring to is Searle's (1980) infamous Chinese Room Argument against computationalism (or the computational theory of mind). I am one of the tiny minority (possibly as few as two) who think Searle's Argument is absolutely right. Computationalism (Dietrich 1990) holds that mental states are just computational states -- not just any computational states, of course, only certain special ones, in particular, those that are sufficient to pass the Turing Test (TT), which calls for the candidate computational system to be able to correspond with us as a pen pal for a lifetime, indistinguishable from a real pen pal. The critical property of computationalism that makes it susceptible to empirical refutation by Searle's thought experiment is the very property that had made it attractive to mind-modellers: its implementation-independence. According to computationalism, all the physical details of the implementation of the right computational states are irrelevant, just as long as they are implementations of the right computational states; for then each and every implementation will have the right mental states. It is this property of implementation-independence -- a property that has seemed to some theorists (e.g. Pylyshyn 1984) to be the kind of dissociation from the physical that might even represent a solution to the mind/body problem -- that Searle exploits in his Chinese Room Argument. He points out that he too could become an implementation of the TT-passing computer program -- after all, the only thing a computer program is really doing is manipulating symbols on the basis of their shapes -- by memorizing and executing all the symbol manipulation rules himself. He could do this even for the (hypothetical) computer program that could pass the TT in Chinese, yet he obviously would not be understanding Chinese under these conditions; hence, by transitivity of implementation-independent properties (or their absence), neither would the (hypothetical) computer implementation be understanding Chinese (or anything). So much for the computational theory of mind.

It's important to understand what Searle's argument does and does not show. It does not show that the TT-passing computer cannot possibly be understanding Chinese. Nothing could show that, because of the other-minds problem (Harnad 1984, 1991): The only way to know that for sure would be to be the computer. The computer might be understanding Chinese, but if so, then that could only be because of some details of its particular physical implementation (that its parts were made of silicon, maybe?), which would flatly contradict the implementation-independence premise of computationalism (and leave us wondering about what's special about silicon). Searle's argument also does not show that Searle himself could not possibly be understanding Chinese under those conditions. But -- unless we are prepared to believe either in the possibility that (1) memorizing and manipulating a bunch of meaningless symbols could induce multiple personality disorder (a condition ordinarily caused only by early child abuse), giving rise to a second, Chinese-understanding mind in Searle, an understanding of which he was not consciously aware (Dyer 1990, Harnad 1990c, Hayes et al 1992), or, even more far-fetched, that (2) memorizing and manipulating a bunch of meaningless symbols could render them consciously understandable to Searle -- the emergence of either form of understanding under such conditions is, by ordinary inductive standards, about as likely as the emergence of clairvoyance (which is likewise not impossible).

So I take it that, short of sci-fi special pleading, Searle's Argument that there is nobody home in there is valid for the very circumscribed special case of any system that is purported to have a mind purely in virtue of being an implementation-independent implementation of a TT-passing computer program. The reason his argument is valid is also clear: Computation is just the manipulation of physically implemented symbols on the basis of their shapes (which are arbitrary in relation to what they can be interpreted to mean); the symbols and symbol manipulations will indeed bear the weight of a systematic interpretation, but that interpretation is not intrinsic to the system (any more than the interpretation of the symbols in a book is intrinsic to the book). It is projected onto the system by an interpreter with a mind (as in the case of the real Chinese pen-pal of the TT-passing computer); the symbols don't mean anything to the system, because a symbol-manipulating system is not the kind of thing that anything means anything to.

There remains, however, the remarkable property of computation that makes it so valuable, namely, that the right computational system will indeed bear the weight of a systematic semantic interpretation (perhaps even the TT). Such a property is not to be sneezed at, and Searle does not sneeze at it. He calls it "Weak Artificial Intelligence" (to contrast it with "Strong Artificial Intelligence," which is the form of computationalism his argument has, I suggest, refuted). The practitioners of Weak AI would be studying the mind -- perhaps even arriving at a true, complete description of it -- using computer models; they could simply never claim that their computer models actually had minds.

Symbolic Oracles

But how can a symbol system mirror most, perhaps all the properties of mind without actually having a mind? Wouldn't the correspondence be too much to be just coincidental? By way of explication, I suggest that we think of computer simulations as symbolic oracles. Consider a computer simulation of the solar system; in principle, we could encode in it all the properties of all the planets and the sun, all the relevant physical laws (discretely approximated, but as closely as we like) such that the solar system simulation could correctly predict and generate all the positions and interactions of the planets far into the future, simulating them in virtual or even real time. Depending on how thoroughly we had encoded the relevant astrophysical laws and boundary conditions, there would be a one-to-one correspondence between the properties of the simulation that was interpretable as a solar system -- let's call it the "virtual" solar system -- and the properties of the real solar system. Yet none of us would, I hope, want to say that there was any, say, motion, or mass, or gravity in the virtual solar system. The simulation is simply a symbol system that is systematically interpretable as if it had motion, mass or gravity; in other words, it is a symbolic oracle.

This is also the correct way to think of a (hypothetical) TT-passing computer program. It would really just be a symbol system that was systematically interpretable as if it were a pen-pal corresponding with someone. This virtual pen-pal may be able to predict correctly all the words and thoughts of the real pen-pal it was simulating, oracularly, till doomsday, but in doing so it is no more thinking or understanding than the virtual solar system is moving. The erroneous idea that there is any fundamental difference between these two cases (the mental model and the planetary model) is, I suggest, based purely on the incidental fact that thinking is unobservable, whereas moving is observable; but gravity is not observable either, and quarks and superstrings even less so; yet none of those would be present in a virtual universe either. For the virtual universe, like the virtual mind, would really just be a bunch of meaningless symbols -- squiggles and squoggles -- that were syntactically manipulated in such a way as to be systematically interpretable as if they thought or moved, respectively. This is certainly stirring testimony to the power of computation to describe and predict physical phenomena and to the validity of the Church/Turing Thesis (Davis 1958; Kleene 1969), but it is no more than that. It certainly is not evidence that thinking is just a form of computation.

So computation is a powerful, indeed oracular, tool for modelling, predicting and explaining planetary motion, life and mind, but it is not a powerful enough tool for actually implementing planetary motion, life or mind, because planetary motion, life and mind are not mere implementation-independent computational phenomena. I will shortly return to the question of what might be powerful enough in its place, but first let me try to tie these considerations closer to the immediate concerns of this conference on artificial life.

Virtual Life: "As If" or Real?

Chris Langton once proposed to me an analogue of the TT, or rather, a generalization of it from artificial mind to artificial life: Suppose, he suggested, that we could encode all the initial conditions of the biosphere around the time life evolved, and, in addition, we could encode the right evolutionary mechanisms -- genetic algorithms, game of life, what have you -- so that the system actually evolved the early forms of life, exactly as it had occurred in the biosphere. Could it not, in principle, go on to evolve invertebrates, vertebrates, mammals, primates, man, and then eventually even Chris and me, having the very conversation we were having at the time in a pub in Flanders -- indeed, could it not go on and outstrip us, cycling at a faster pace through the same experiences and ideas Chris and I would eventually go on to arrive at in real time? And if it could do all that, and if we accept it as a premise (as I do) that there would be not one property of the real biosphere, or of real organisms, or of Chris or me, that would not also be present in the virtual world in which all this virtual life, and eventually these virtual minds, including our own, had "evolved," how could I doubt that the virtual life was real? Indeed, how could I even distinguish them?

Well the answer is quite simple, as long as we don't let the power of hermeneutics loosen our grip on one crucial distinction: the distinction between objects and symbolic descriptions of them. There may be a one-to-one correspondence between object and symbolic description, a description that is as fine-grained as you like, perhaps even as fine-grained as can be. But the correspondence is between properties that, in the case of the real object, are what they are intrinsically, without the mediation of any interpretation, whereas in the case of the symbol description, the only "objects" are the physical symbol tokens and their syntactically constrained interactions; the rest is just our interpretation of the symbols and interactions as if they were the properties of the objects they describe. If the description is complete and correct, it will always bear the full weight of that interpretation, but that still does not make the corresponding properties in the object and the description identical properties; they are merely computationally equivalent under the mediation of the interpretation. This should be only slightly harder to see in the case of a dynamic simulation of the universe than it is in the case of a book full of static sentences about the universe.

In the case of real heat and virtual heat or real motion and virtual motion the distinction between identity and formal equivalence is clear. A computer simulation of a fire is not really hot and nothing is really moving in a computer simulation of planetary motion. In the case of thinking I have already argued that we have no justification for claiming an exception merely because thinking is unobservable (and besides, Searle's "periscope" shows us that we wouldn't find thinking in there even if we became the simulation and observed for ourselves what is unobservable to everyone else). What about life? But we have already seen that life -- once we bracket the mind/body problem -- is not different from any other physical phenomenon, so virtual life is no more alive than virtual planetary motion moves or virtual gravity attracts. Chris Langton's virtual biosphere, in other words, would be just another symbolic oracle: yet another bunch of systematically interpretable squiggles and squoggles: Shall we call this "Weak Artificial Life"? (Sober 1992, as I learned after writing this paper, had already made this suggestion at the Artificial Life II meeting in 1990).

Sensorimotor Transduction, Robotics, The "Total Turing Test," and Symbol Grounding

I did promise to return to the question of what more might be needed to implement life if computation, for all its power and universality, is not strong enough. Again, we return to the Chinese Room Argument for a hint, but this time we are interested in what kind of system Searle's argument cannot successfully show to be mindless: We do not have to look far. Even an optical transducer is immune to Searle, for if someone claimed to have a system that could really see (as opposed to being merely interpretable as if it could see), Searle's "periscope" would already fail (Harnad 1989). For if Searle tried to implement the putative "seeing" system without seeing (as he had implemented the putative "understanding" system without understanding), he would have only two choices. One would be to implement only the output of the transducer (if we can assume that its output would be symbols) and whatever symbol manipulations were to be done on that output, but then it would not be surprising if Searle reported that he could not see, for he would not be implementing the whole system, just a part of it. All bets are off when only parts of systems can be implemented. Searle's other alternative would be to actually look at the system's scene or screen in implementing it, but then, alas, he would be seeing. Either way, the Chinese Room strategy does not work. Why? Because mere optical transduction is an instance of the many things there are under the sun -- including touch, motion, heat, growth, metabolism, photosynthesis, and countless other "analog" functions -- that are not just implementations of implementation-independent symbol manipulations. And only the latter are vulnerable to Searle's Argument.

This immediately suggests a more exacting variant of the Turing Test -- what I've called the Total Turing Test (TTT) -- which, unlike the TT, is not only immune to Searle's Argument but reduces the level of underdetermination of mind-modelling to the normal level of underdetermination of scientific theory (Harnad 1989, 1991). The TT clearly has too many degrees of freedom, for we all know perfectly well that there's a lot more that people can do than be pen-pals. The TT draws on our linguistic capacity, but what about all our robotic capacities, our capacity to discriminate, identify and manipulate the objects, events and states of affairs in the world we live in? Every one of us can do that; so can animals (Harnad 1987; Harnad et al. 1991). Why should we have thought that a system deserved to be assumed to have a mind if all it could generate was our pen-pal capacity? Not to mention that there is good reason to believe that our linguistic capacities are grounded in our robotic capacities. We don't just trade pen-pal symbols with one another; we can each also identify and describe the objects we see, hear and touch, and there is a systematic coherence between how we interact with them robotically and what we say about them linguistically.

My own diagnosis is that the problem with purely computational models is that they are ungrounded. There may be symbols in there that are systematically interpretable as meaning "cat," "mat" and "the cat is on the mat," but in reality they are just meaningless squiggles and squoggles apart from the interpretation we project onto them. In an earlier conference in New Mexico (Harnad 1990a) I suggested that the symbols in a symbol system are ungrounded in much the same way the symbols in a Chinese-Chinese dictionary are ungrounded for a non-speaker of Chinese: He could cycle through it endlessly without arriving at meanings unless he already had grounded meanings to begin with (as provided by a Chinese-English dictionary, for an English speaker). Indeed, translation is precisely what we are doing when we interpret symbol systems, whether static or dynamic ones, and that's fine, as long as we are using them only as oracles, to help us predict and explain things. For that, their systematic interpretability is quite sufficient. But if we actually want them to implement the things they predict and explain, they must do a good deal more. At the very least, the meanings of the symbols must somehow be grounded in a way that is independent of our projected interpretations and in no way mediated by them.

The TTT accordingly requires the candidate system, which is now a robot rather than just a computer, to be able to interact robotically with (i.e., to discriminate, identify, manipulate and describe) the objects, events and states of affairs that its symbols are systematically interpretable as denoting, and it must be able to do it so its symbolic performance coheres systematically with its robotic performance. In other words, not only must it be capable, as any pen-pal would be, of talking about cats, mats, and cats on mats indistinguishably from the way we do, but it must also be capable of discriminating, identifying, and manipulating cats, mats, and cats on mats exactly as any of us can; and its symbolic performance must square fully with its robotic performance, just as ours does (Harnad 1992).

The TT was a tall order; the TTT is an even taller one. But note that whatever candidate successfully fills the order cannot be just a symbol system. Transduction and other forms of analog performance and processing will be essential components in its functional capacity, and subtracting them will amount to reducing what's left to those mindless squiggles and squoggles we've kept coming up against repeatedly in this discussion. Nor is this added performance capacity an arbitrary demand. The TTT is just normal empiricism. Why should we settle for a candidate that has less than our full performance capacities? The proper time to scale the model down to capture our handicaps and deficits is only after we are sure we've captured our total positive capacity (with the accompanying hope that a mind will piggy-back on it) -- otherwise we would be like automotive (reverse) engineers (i.e., theoretical engineers who don't yet know how cars work but have real cars to study) who were prepared to settle for a functional model that has only the performance capacities of a car without moving parts, or a car without gas: The degrees of freedom of such "handicapped" modelling would be too great; one could conceivably go on theory-building forever without ever converging on real automobile performance capacity that way.

A similar methodological problem unfortunately also affects the TTT modelling of lower organisms: If we knew enough about them ecologically and psychologically to be able to say with any confidence what their respective TTT capacities were, and whether we had captured them TTT-indistinguishably, lower organisms would be the ideal place to start; but unfortunately we do not know enough, either ecologically or psychologically (although attempts to approximate the TTT capacities of lower organisms will probably still have to precede or at least proceed apace with our attempts to capture human TTT capacity).

The TTT Versus the TTTT

Empiricists might want to counter that the proper degrees of freedom for mind modelling are neither those of the TT nor the TTT but the TTTT (Total-Total Turing Test), in which the candidate must be empirically indistinguishable from us not only in all of its macrobehavioral capacities but also in all of its microstructural (including neural) properties (Churchland 1984, 1986, 1989). I happen to think that the TTTT would be supererogatory, even overly constraining, and that the TTT already narrows the degrees of freedom sufficiently for the branch of reverse engineering that mind-modelling belongs to. There is still a kind of implementation-independence here too, but not a computationalist kind: There is no reason to believe that biology has exhausted all the possibilities of optical transduction, for example. All optical transducers must transduce light, to be sure, but apart from that, there is room for a lot more possibilities along the continuum on which the human retina and the Limulus ommatidia represent only two points.

The world of objects and the physics of transducing energy from them provide the requisite constraints for mind-modelling, and every solution that manages to generate our TTT capacity within those constraints has (by my lights) equal claim to our faith that it has mental states -- I don't really see the one that is TTTT-indistinguishable from us as significantly outshining the rest. My reasons for believing this are simple: We are blind to Turing indistinguishable differences (that's why there's an other-minds problem and a mind/body problem). By precisely the same token, the Blind Watchmaker is likewise blind to such differences. There cannot have been independent selection pressure for having a mind, since selection pressure can operate directly only on TTT capacity.

Yet a case might be made for the TTTT if the capacity to survive, reproduce and propagate one's genes is an essential part of our TTT capacity, for that narrows down the range of eligible transducers still further, and these are differences that evolution is not blind to. In this case, the TTTT might pick out microstructural features that are too subtle to be reflected in individual behavioral capacity, and the TTT look-alikes lacking them might indeed lack a mind (cf. Morowitz 1992).

My own hunch is nevertheless that the TTT is strong enough on its own (although neuroscience could conceivably give us some clues as to how to pass it), and I'm prepared to extend human rights to any synthetic candidate that passes it, because the TTT provides the requisite constraints for grounding symbols, and that's strong enough grounds for me. I doubt, however, that TTT capacity can be second-guessed a priori, even with the help of symbolic oracles. Perhaps this is the point where we should stop pretending that mind-modellers can bracket life and that life-modelers can bracket mind. So far, only living creatures seem to have minds. Perhaps the constraints on the creation of synthetic life will be relevant to the constraints on creating synthetic minds.

These last considerations amount only to fanciful speculation, however; the only lesson Artificial Life might take from this paper is that it is a mistake to be too taken with symbol systems that display the formal properties of living things. It is not that only natural life is possible; perhaps there can be synthetic life too, made of radically different materials, operating on radically different functional principles. The only thing that is ruled out is "virtual" or purely computational life, because life (like mind and matter) is not just a matter of interpretation.

Coda: Analog "Computation"

Let me close with a remark on analog computation. Searle's Chinese Room Argument applies only to computation in the sense of finite, discrete symbol manipulation. Searle cannot implement a transducer because the transduction of physical energy is not just the manipulation of finite discrete symbols. Searle would even be unable to implement a parallel, distributed system like a neural net (though he could implement a discrete serial simulation of one; Harnad 1993). Indeed, every form of analog computation is immune to Searle's Argument. Is analog computation also immune to the symbol grounding problem? I am inclined to say it is, if only because "symbols" usually mean discrete physically tokened symbols. Some people speak of "continuous symbols," and even suggest that the (real) solar system is an (analog) computer if we choose to use it that way (MacLennan 1987, 1988, in press a, b, c). We are here getting into a more general (and I think vaguer) sense of "computing" that I don't think says much about either life or mind one way or the other. For if the solar system is computing, then everything is computing, and hence whatever the brain -- and any synthetic counterpart of it -- is actually doing, it too is computing. When "implementation independence" becomes so general as to mean obeying the same differential equations, I think the notion that a system is just the implementation of a computation becomes rather uninformative. So take my argument against the overinterpretation of virtual life to be applicable only to finite, discrete symbol systems that are interpretable as if they were living, rather than to systems that obey the same differential equations as living things, which I would regard as instances of synthetic rather than virtual life.


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