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The thesis at issue here is this: whether or not the Argument from Irreversibility presented by Bringsjord in his paper “Cognition is not Computation: The Argument From Irreversibility” ([1]) is sound or unsound. I believe it to be unsound, based on a simple logical argument: 1) If the Argument from Irreversibility is sound, then nothing based on irreversible physical processes is computation 2) What computers do is based on irreversible physical processes 3) If the Argument from Irreversibility is sound, what computers do is not computation (1, 2) 4) What computers do is computation 5) The Argument from Irreversibility is unsound. (3, 4) This logic is clearly valid, so the argument rests on the truth of premises 1, 2, and 4. Premise 4, I hope, is uncontroversial – if it were false, then computer scientists would have a lot more to worry about than the veracity of Computationalism! I believe Premise 1 is obvious given the arguments in the paper [1], but it still warrants some recap and explanation. The Argument from Irreversibility hinges on the assertion that cognition, including consciousness, is not reversible. Bringsjord follows two main paths in backing up this assertion: the first, phenomenological one attempts to demonstrate that reversal of consciousness is logically impossible because we cannot conceive of it, while the second contends that the actual physical processes going on in the brain are not physically reversible, due in large part to increasing entropy, and therefore whatever these processes entail, such as consciousness according to the Computationalist view, is not reversible. It is this latter view that is of relevance here (I am assuming the first to lead nowhere, because even as Bringsjord has admitted, “We have no outright proof that Pconsciousness is irreversible in this strong modal sense” [1]). The latter view entails that nothing based on irreversible physical processes is computation, by the same logic that leads Bringsjord to conclude that consciousness cannot be computation if it’s based on irreversible physical processes (since computation is reversible). If the soundness of the Argument from Irreversibility requires this physics-based assertion to be true, which seems to be the case given the bulk of the argument in [1], then Premise 1 above is certainly sound. Premise 2 is based on the fact that computers are constructed of silicon chips, and operate by pushing electrons around inside their wiring – a process that is certainly irreversible in the physical sense. Computer processors dissipate large amounts of heat, for example, which signifies a lack of thermodynamic equilibrium, and therefore a system that cannot be reversed in the sense that Bringsjord requires of it. Bringsjord, however, would probably have a ready response to this, hinging around Premise 2: That, yes, today’s computers are based on irreversible physical processes, but the significant aspects of those computers, viz. the information flow within them, is reversible, and is translated faithfully to the physical level. That Bringsjord would respond this way is implied by passages such as this: Two different and determinate levels may be addressed by Theorem 1 [reversibility of computation] (and the like): perhaps there is the purely mathematical level of the theorem itself, and then perhaps also what might be called the “logic gate level” of computer engineering…. Computer engineers have on hand physical instantiations of Turing Machines…. [B]oth theoretical computers science and computer engineering is [sic] constrained and informed by Theorem 1 and its relatives. [1, in footnotes] This “logic gate level” seems to be the level of abstraction at which reversibility, and computation, still hold, and that which is faithfully reproduced (that is, none of the relevant behavior is lost) by the physical (yet nonreversible) instantiation. I agree with this; however, there is a similar “way out” for consciousness and the mind: perhaps there is a layer above the physical representation of the mind which faithfully reproduces all relevant aspects (even p-consciousness) of the physical instantiation, and which is reversible and therefore computable. Dennett [2] (quoted by Bringsjord in [1]) seems to imply such a view: The central doctrine of cognitive science is that there is a level of analysis, the information-processing level, intermediate between the phenomenological level (the personal level, or the level of consciousness) and the neurophysiological level (p195). Bringsjord also concisely represents this argument against him in [1] as: “‘…what you fail to appreciate is that at the information-processing level to which Dennet has drawn our attention consciousness is reversible…. you have surreptitiously moved at the same time to a level beneath this level.’” And then, he provides his argument against this whole concept -- representing the mind and consciousness -- and therefore the mind’s “way out” of the problem of physical irreversibility: The problem with this objection is that it conveniently ignores the fact that computationalism is wed not only to information processing, but also to agent materialism, the view that cognizers are physical things, and that therefore cognition is a physical process. In light of this, introducing at least elementary considerations from physics …[is] unavoidable. [1] Bringsjord stresses this idea elsewhere (“…Computationalists as a rule hold that minds are physical things” in a footnote in [1]). Therefore, it seems that Bringsjord is asserting that using an abstract representation of the mind rather than regarding the actually physical “stuff” is not Computationalism. However, I believe that this assertion is an unfair characterization of Computationalism, and even a straw man attack. The reason I believe it to be unfair is that an abstract representation of a physical thing, which is very close to the physical thing in question, and that contains all the relevant features of the physical thing, can be regarded as equivalent to that physical thing for our discussion. In fact, Bringsjord uses exactly this idea when formulating his characterization of Computationalism through Proposition 1’ (from [1]): Proposition 1'. x(Px ^ x is conscious from ti to ti+k y(My ^ x = y ^ Cj├y Cj+1├y ├y Cj+p)), where this computation is identical to the consciousness x enjoys through [ti, ti+k]. Notice the x = y, where x is a person and y is a Turing machine (or a neural network). Persons (or their brains) are certainly physical things (assuming agent materialism), and Turing machines are certainly abstract concepts. So in his own characterization of Computationalism, Bringsjord is using the principle described above, where abstract concepts and their accurate physical instantiations can be regarded as equal. In this case, why can’t Computationalists deal with an accurate abstract representation of the physical mind (for example, an insanely complex kind of neuron simulation network), which is surely computation, and reversible, in the same way that computer engineers use the “logic gate level” abstraction of the irreversible physical processes in the silicon of computers? I can’t see any response except that “an accurate abstract representation of the mind which involves all relevant physical aspects is impossible” – something for which Bringsjord doesn’t make an argument in his paper. And it seems to me that if the mind is purely physical, and is represented completely by our neurons and their interactions, then a “neuron simulator” kind of neural-net approach (such physically accurate simulators are in the works, such as CalTech’s GENESIS) would capture all relevant aspects of the operation of the physical mind, and therefore would also capture the mind’s consciousness. I see this notion fitting perfectly in Computationalism. References: [1] Bringsjord, Selmer. “Cognition is not Computation: The Argument From Irreversibility.” Available from http://www.rpi.edu/~brings/pai.html. [2] Dennett, D. (1993) ``Review of John Searle's The Rediscovery of the Mind," Journal of Philosophy 90.4: 193-205