Multiple realizability - AbsoluteAstronomy.com

Multiple realizability, in philosophy of mind

Philosophy of mind

Philosophy of mind is a branch of philosophy that studies the nature of the mind, mental events, mental functions, mental properties, consciousness and their relationship to the physical body, particularly the brain. The mind-body problem, i.e...

, 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

Hilary Putnam

Hilary Whitehall Putnam is an American philosopher, mathematician and computer scientist, who has been a central figure in analytic philosophy since the 1960s, especially in philosophy of mind, philosophy of language, philosophy of mathematics, and philosophy of science...

and Jerry Fodor

Jerry Fodor

Jerry Alan Fodor is an American philosopher and cognitive scientist. He holds the position of State of New Jersey Professor of Philosophy at Rutgers University and is the author of many works in the fields of philosophy of mind and cognitive science, in which he has laid the groundwork for the...

, put it forth as an argument against reductionist

Reductionism

Reductionism can mean either an approach to understanding the nature of complex things by reducing them to the interactions of their parts, or to simpler or more fundamental things or a philosophical position that a complex system is nothing but the sum of its parts, and that an account of it can...

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

Eliminative materialism

Eliminative materialism is a materialist position in the philosophy of mind. Its primary claim is that people's common-sense understanding of the mind is false and that certain classes of mental states that most people believe in do not exist...

. The same arguments from multiple realizability were also used to defend many versions of functionalism

Functionalism (philosophy of mind)

Functionalism is a theory of the mind in contemporary philosophy, developed largely as an alternative to both the identity theory of mind and behaviourism. Its core idea is that mental states are constituted solely by their functional role — that is, they are causal relations to other mental...

, 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 recently noted that the multiple realizability of the mental is a thesis Turing held at least ten years before the authors to whom it is usually attributed to did. In 1950 Turing expressed the multiple realizability of the mental in this way:

The 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.

Putnam's formulation

A classic example of the thesis of multiple realizability is to be found in several papers published by Hilary Putnam

Hilary Putnam

in the late 1960s. In these papers, he argued that, contrary to the famous claim of type-identity theory

Type physicalism

Type physicalism is a physicalist theory, in philosophy of mind. It asserts that mental events can be grouped into types, and can then be correlated with types of physical events in the brain...

, 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

Neuroanatomy

Neuroanatomy is the study of the anatomy and organization of the nervous system. In contrast to animals with radial symmetry, whose nervous system consists of a distributed network of cells, animals with bilateral symmetry have segregated, defined nervous systems, and thus we can begin to speak of...

and neurophysiology

Neurophysiology

Neurophysiology is a part of physiology. Neurophysiology is the study of nervous system function...

. 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.

Fodor's generalization

Jaegwon Kim

Jaegwon Kim is a Korean American philosopher currently working at Brown University. He is best known for his work on mental causation and the mind-body problem. Key themes in his work include: a rejection of Cartesian metaphysics, the limitations of strict psychophysical identity, supervenience,...

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

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

Neuroplasticity

Neuroplasticity is a non-specific neuroscience term referring to the ability of the brain and nervous system in all species to change structurally and functionally as a result of input from the environment. Plasticity occurs on a variety of levels, ranging from cellular changes involved in...

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

Ernest Nagel

Ernest Nagel was a Czech-American philosopher of science. Along with Rudolf Carnap, Hans Reichenbach, and Carl Hempel, he is sometimes seen as one of the major figures of the logical positivist movement....

'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

Universal Turing machine

In computer science, a universal Turing machine is a Turing machine that can simulate an arbitrary Turing machine on arbitrary input. The universal machine essentially achieves this by reading both the description of the machine to be simulated as well as the input thereof from its own tape. Alan...

Against the early version

Early objections to multiple realizability were limited to the narrow, "across structures-type" version. Starting with David Kellogg Lewis

David Kellogg Lewis

David Kellogg Lewis was an American philosopher. Lewis taught briefly at UCLA and then at Princeton from 1970 until his death. He is also closely associated with Australia, whose philosophical community he visited almost annually for more than thirty years...

, 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

Intertheoretic reduction

In philosophy of science, intertheoretic reduction occurs when one theory makes predictions that perfectly or almost perfectly match the predictions of a second theory. Newtonian mechanics, for example, can be reduced to general relativity for velocities far less than c....

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.

Against the general version

However, reductionists (perhaps the most recent examples being Bechtel

William Bechtel

William Bechtel is a professor of Philosophy in the Department of Philosophy and the Science Studies Program at the University of California, San Diego. He was a Professor of Philosophy at Washington University in St. Louis from 1994 until 2002...

and Mundale) reply that this is simply not empirically plausible. In order 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 (or homology

Homology (biology)

Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...

) 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).

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 characteristics that are shared by two or more species but that are not inherited from a common ancestor. Human arms and bird wings are homologous, while bird wings and bat wings (even though they strongly resemble each other) are not. 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 homoplasious brain structures and determine whether some "psychological processes or functions might be 'constructed' from different material" and supported by different brain structures just as the wings of bats and those of birds are constructed from different materials. The emergence of the same identical brain structures by evolutionary convergence

Convergent evolution

Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...

would, perhaps, provide some (weak) evidence against multiple realizability, since it is highly improbable that this could actually happen.

It would be weak evidence because it would still not eliminate the possibility of the existence of mental states identical to ours in non-carbon based life forms on other planets or in machines.

Kim's argument

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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