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Thesis in the philosophy of mind
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. Philosophers of mind have used multiple realizability to argue that mental states are not the same as — and cannot be reduced to — physical states. They have also used it to defend or criticize many versions of functionalism, especially machine state functionalism.

The multiple realizability thesis in the philosophy of mind is the thesis that the same mental state can be realized by different physical states; another way of putting it is that there is a many to one mapping from physical states to mental states. Multiple realizability in general is not restricted to the multiple realizability of mental states. Many kinds of things can be realized by numerous physical devices. A wide variety of physical devices can serve as corkscrews, for example. Mental states can also be realized in a variety of ways. Just as the logical states of a Turing machine can be realized by different structural states in different mechanisms, so, by analogy, the mental states of a human being can be realized by different physical states in different individuals. Pain, for example, is correlated with different physical states of the nervous system in different organisms, but the organisms all experience the mental state of "being in pain." Mental states have been claimed to be multiply realizable not only across species and between individuals but also within individuals. At different times, the same individual may realize the same mental states in physically different forms. Neural plasticity — the fact that areas of the brain can assume the functions of other parts that have been damaged as the result of traumatic injury, pathology, natural biological development, or other processes — has long been considered to be an example. But so are more mundane facts about neurophysiology, such as the fact that neurons die and connections between them are rewired. Gualtiero Piccinini differentiates three related properties: variable realizability, multiple realizability, and medium independence. A property is variably realizable if it can be instantiated by different realizers. For example, both a winged corkscrew and a waiter's corkscrew have the property of removing corks and do so through the same mechanism — a screw and pull mechanism. Because the mechanism is fundamentally unchanged, the property is variably realizable. For a property to be multiply realizable, the property must be able to be instantiated by different realizers and different mechanisms. The classic spring mousetrap and the glue mousetrap instantiate the same property, the ability to trap mice, but they do so through different mechanisms. As such, the property is multiply realizable. A property is medium independent if it can be instantiated by different realizers and different mechanisms and if the inputs and outputs of the mechanisms are also multiply realizable. A mousetrap is not medium independent; it must take a mouse as an input. A computer, though, is medium independent. A computer can be constructed from different parts assembled into different mechanisms and can take different types of inputs and outputs. In typical digital computers, the inputs and outputs are voltages, but in quantum computers, the inputs and outputs would be different.

Multiple realizability has been used as an argument against type identity theory, against reductionist theories of mind in general, for functionalist theories of mind, and even against functionalist theories of mind. Starting in the 1960s, Hilary Putnam used multiple realizability as an argument against type identity theory. Specifically, Putnam noted that the multiple realizability of pain entails that, contrary to type identity theory, pain is not identical to C fibre firing. More generally, multiple realizability shows that psychological attributes are not the same as physical attributes. Psychological attributes, rather, are disjunctions of physical attributes. In 1988, Hilary Putnam used multiple realizability to argue against functionalism. Noting that functionalism is essentially a watered down reductionist or identity theory in which mental kinds are ultimately identified with functional kinds, Putnam argues that mental kinds are probably multiply realizable over functional kinds. The same mental state or property can be implemented or realized by different states of a universal Turing machine.

Putnam cites examples from the animal kingdom as evidence for the multiple realizability of mental states. Some philosophers refer to this argument — that multiple realizability is much more likely than reductionism — as the likelihood argument.

Putnam also formulates a complementary argument based on functional isomorphism. He defines 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." Two computers, for example, are functionally isomorphic if the sequential relations among states in one are exactly mirrored by those in the other. A computer made of electrical components and a computer made of cogs and wheels can be functionally isomorphic even though they are constitutionally different.

Jaegwon Kim argues that disjunction — the idea that the physical realization of a particular mental state is not a particular physical state but the disjunction of the physical states that realize that mental state — creates problems for multiple realizability. Putnam also argued against this "disjunctive" possibility in later work.

Kim has also argued against non reductive physicalism on the grounds that it violates the causal closure of the physical, which assumes 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 Kim is right, then the options seem to be either reduction or elimination.

One criticism of multiple realizability is that any theory that attempts to address 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. writing: "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."