Multiple realizability, in the philosophy of mind, is the thesis that the same mental property, state, or event can be implemented by different physical properties, states or events. The idea is widely believed to have its roots in the late 1960s and early 1970s when a number of philosophers, most prominently Hilary Putnam and Jerry Fodor, put it forth as an argument against reductionist accounts of the relation between mental and physical kinds. In short, a theory of mind that includes multiple realizability allows for the existence of strong AI. The original targets of these arguments were the type-identity theory and eliminative materialism. The same arguments from multiple realizability were also used to defend many versions of functionalism, especially Machine state functionalism.
In recent years, however, multiple realizability has been used as a weapon to attack the very theory that it was originally designed to defend. Functionalism has consequently fallen out of vogue as a dominant theory in the philosophy of mind. The dominant theory ("received view" in the words of Lepore and Pylyshyn) in modern philosophy of mind is a sort of generic non-reductive physicalism and one of its central pillars is the hypothesis of multiple realizability.
In addition, Restrepo noted in 2009 that the multiple realizability of the mental is a thesis Turing held at least ten years before the usually attributed authors described it. In 1950 Turing expressed the multiple realizability of the mental in this way:
The [Babbage Engine's] storage was to be purely mechanical, using wheels and cards.
The fact that Babbage's Analytical Engine was to be entirely mechanical will help us rid ourselves of a superstition. Importance is often attached to the fact that modern digital computers are electrical, and the nervous system is also electrical. Since Babbage's machine was not electrical, and since all digital computers are in a sense equivalent, we see that this use of electricity cannot be of theoretical importance. [...] If we wish to find such similarities we should look rather for mathematical analogies of function.
A classic example of the thesis of multiple realizability is to be found in several papers published by Hilary Putnam in the late 1960s. In these papers, he argued that, contrary to the famous claim of type-identity theory, it was not true that "pain is identical to C-fibre firing." It is possible that pain corresponds to, or is at least correlated with, completely different physical states of the nervous system in different organisms and yet they all experience the same mental state of "being in pain." Putnam cited numerous examples from all over the animal kingdom to illustrate his thesis. Is it likely that the brain structures of all mammals, reptiles, birds, amphibians and molluscs realize pain, or other mental states, in exactly the same way? Do they even have the same brain structures? Clearly not, if we are to believe the evidence furnished by comparative neuroanatomy and neurophysiology. How is it possible then that they can share the same mental states and properties? The answer had to be that these mental kinds were realized by different physical states in different species. Putnam then took his argument a step further, asking about such things as the nervous systems of alien beings, artificially-intelligent robots and silicon-based life forms. Should such hypothetical entities be considered a priori incapable of experiencing pain just because they did not possess the same neurochemistry as humans? Putnam concluded that type-identity and other reductive theorists had been making an extremely "ambitious" and "highly implausible" conjecture which could be disproved with just one example of multiple realizability. This is sometimes referred to as the likelihood argument.
Putnam also formulated a complementary argument based on, what he called, functional isomorphism. He defined the concept in these terms: "Two systems are functionally isomorphic if there is a correspondence between the states of one and the states of the other that preserves functional relations." So, in the case of computers, two machines are functionally isomorphic if and only if the sequential relations among states in the first are exactly mirrored by the sequential relations among states in the other. Therefore, a computer made out of silicon chips and a computer made out of cogs and wheels can be functionally isomorphic but constitutionally diverse. Functional isomorphism implies multiple realizability. This is sometimes referred to as an "a priori argument".
Jerry Fodor, Putnam and others immediately noted that, along with being a very effective argument against type-identity theories, multiple realizability implied that any low-level explanation of higher-level mental phenomena would be insufficiently abstract and general. Functionalism, which attempts to identify mental kinds with functional kinds that are characterized exclusively in terms of causes and effects, abstracts from the physico-chemical level of microphysics and hence seemed to be a more suitable alternative explanation of the relation between mind and body. In fact, there are many functional kinds such as mousetraps, software and bookshelves which are multiply realized at the physical level.
Jaegwon Kim took up the challenge of responding to the problems posed by multiple realizability for reductionist theories by suggesting that the physical realization base of a particular mental state was not a particular physical state but the disjunction of the physical states which realize it. Jerry Fodor replied to this objection by formulating a generalization of the multiple realizability thesis. According to Fodor, multiple realizability was not just something that occurred "across physical structure-types" but was a phenomenon that could occur even within the same token system (such as an organism). At different times, the same organism may realize type-identical mental kinds in physically different forms. (This thesis was later given some empirical support with the discovery of the relative plasticity of the human brain).
Fodor used this generalized multiple realizability thesis to argue against reductionism of the mind and of the special sciences. The key to Fodor's argument is that, in his characterization of reductionism, all mental kind predicates in an ideal and completed psychology must correspond with a physical kind predicate in an ideal and completed physics. He suggested taking Ernest Nagel's theory of reduction, which insisted on the derivability of all terms in the theory to be reduced from terms in the reducing theory and the bridging laws, as the canonical theory of reduction. Given generalized multiple realizability, the physical science part of these psychophysical bridge laws will end up being a, possibly infinite, disjunction of all the terms referring to possible physical realizations of a mental kind. This disjunction cannot be a kind-predicate and therefore the entire statement cannot be a law of physics. The special sciences cannot be reduced to physics in this way, according to Fodor.
Later on, in 1988, Hilary Putnam applied the argument from Fodor's generalized version of multiple realizability to argue against functionalism itself, including, and above all, his own version of functionalism, machine state functionalism. Noting that functionalism is essentially a watered-down reductionist or identity theory in which mental kinds are ultimately identified with functional kinds, Putnam argued that mental kinds were probably multiply realizable over functional kinds. The same mental state or property could be implemented or realized by different states of a universal Turing machine.
Objections and responses
Early objections to multiple realizability were limited to the narrow, "across structures-type" version. Starting with David Kellogg Lewis, many reductionists argued that it is very common, perhaps the rule, in actual scientific practice to reduce one theory to another by way of "local" and structure-specific reductions. A frequently cited example of this sort of intertheoretic reduction is the case of temperature from classical thermodynamics. Temperature is identical to mean molecular kinetic energy. But this is only true of temperature in a gas. Temperature in a solid is identical to mean maximal molecular kinetic energy, because the molecules of a solid are more restricted in their movements. Temperature in a plasma is something of a mystery, since the molecules of a plasma are torn apart. Therefore, temperature, in classical thermodynamics is multiply realized in a wide diversity of microphysical states.
One common defence of multiple realizability in the literature, however, is that any such response which attempts to address the problem of the possibility of generalized multiple realizability must necessarily be so "local" and "context" specific in nature, referring exclusively to a certain token system of a certain structure-type at a certain time, that its reductions would be incompatible with even a minimally acceptable degree of generality in scientific theorizing. This problem is well illustrated by the controversial question of the plasticity of the human brain. Neural plasticity consists simply in the fact that different areas of the brain can, and often do, take over the functions of other parts which have been damaged as the result of traumatic injury, pathology, natural biological development and other processes. Any psychology which is narrowed down sufficiently to handle this level of multiple realizability will almost certainly not be general enough to capture the generalizations needed to explain only human psychology.
However, reductionists (perhaps the most recent examples being Bechtel and Mundale) reply that this is simply not empirically plausible. To conduct research and carry out experiments in the neurosciences some universal consistencies must either exist or be assumed to exist in brain structures. It is the similarity (produced by homology or convergent evolution) of brain structures which allows us to generalize across species. If multiple realizability (especially the generalized form) were an empirical fact, then results from experiments conducted on one species of animal (or one organism) would not be meaningful or useful when generalized to explain the behavior or other characteristics of another species (or organism of the same species; or in the generalized form, even the same organism).
Sungsu Kim has recently responded to this objection by pointing to the important distinction between homology of brain structures and homoplasy. Homologies are any characteristics of physiology, morphology, behavior or psychology that are shared by two or more species and that are inherited from a common ancestor. Homoplasies are similar or identical characteristics that are shared by two or more species but that are not inherited from a common ancestor, having evolved independently. The feet of ducks and platypuses are a good example of homoplasy, while the hands of humans and chimps are a good example of homology. The fact that brain structures are homologous is no evidence either for or against multiple realizability. The only way to really empirically test the thesis of multiple realizability would be to examine brain structures and determine whether some homoplasious "psychological processes or functions might be 'constructed' from different material" and supported by different brain structures just as the flight capacities of bats and birds emerge from different morphophysiologies. The emergence of similar behavioral outputs or psychological functions brought about by similar or identical brain structures in convergent evolutive lineages would provide some evidence against multiple realizability, since it is highly improbable that this would happen, if not for constrains on the type of physical system that can realize mental phenomena. This would, however, still not completely refute the possibility of realizibility of mental states in radically different physical systems such as non-carbon based life forms or in machines.
Jaegwon Kim has recently argued against non-reductive physicalism on the grounds that it violates the causal closure of the physical. Roughly, the idea is that physics provides a full explanation of physical events. If mental properties are causally efficacious, they must either be identical to physical properties, or there must be widespread overdetermination. The latter is often held to be either unlikely or even impossible on conceptual grounds. If this is right, then the options seem to be either reduction or elimination.
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