Multiple realizability

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.

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