Twin Earth is a thought experiment proposed by philosopher Hilary Putnam in his papers "Meaning and Reference" (1973) and "The Meaning of 'Meaning (1975). It is meant to serve as an illustration of his argument for semantic externalism, or the view that the meanings of words are not purely psychological. The Twin Earth thought experiment was one of three examples that Putnam offered in support of semantic externalism, the other two being what he called the Aluminum-Molybdenum case and the Beech-Elm case. Since the publication of these cases, numerous variations on the thought experiment have been proposed by philosophers.
The thought experiment
Putnam's original formulation of the experiment was this: We begin by supposing that elsewhere in the universe there is a planet exactly like Earth in virtually all aspects, which we refer to as "Twin Earth". (We should also suppose that the relevant surroundings are exactly the same as for Earth; it revolves around a star that appears to be exactly like our sun, and so on). On Twin Earth, there is a Twin equivalent of every person and thing here on Earth. The one difference between the two planets is that there is no water on Twin Earth. In its place there is a liquid that is superficially identical, but is chemically different, being composed not of H2O, but rather of some more complicated formula which we abbreviate as "XYZ". The Twin Earthlings who refer to their language as "English" call XYZ "water". Finally, we set the date of our thought experiment to be several centuries ago, when the residents of Earth and Twin Earth would have no means of knowing that the liquids they called "water" were H2O and XYZ respectively. The experience of people on Earth with water and that of those on Twin Earth with XYZ would be identical.
Now the question arises: when an Earthling (or Oscar for simplicity's sake) and his twin on Twin Earth say 'water', do they mean the same thing? (The twin is also called 'Oscar' on his own planet, of course. Indeed, the inhabitants of that planet call their own planet 'Earth'. For convenience, we refer to this putative planet as 'Twin Earth', and extend this naming convention to the objects and people that inhabit it, in this case referring to Oscar's twin as Twin Oscar.) Ex hypothesi, they are in identical psychological states, with the same thoughts, feelings, etc. Yet, at least according to Putnam, when Oscar says 'water', the term refers to H2O, whereas when Twin Oscar says 'water' it refers to XYZ. The result of this is that the contents of a person's brain are not sufficient to determine the reference of terms they use, as one must also examine the causal history that led to this individual acquiring the term. (Oscar, for instance, learned the word 'water' in a world filled with H2O, whereas Twin Oscar learned 'water' in a world filled with XYZ.)
This is the essential thesis of semantic externalism. Putnam famously summarized this conclusion with the statement that meanings' just ain't in the head."
Criticism
In his original article, Putnam had claimed that the reference of the twins' "water" varied even though their psychological states were the same. Tyler Burge subsequently argued in "Other Bodies" (1982) that the twins' mental states are different: Oscar has the concept H2O, while Twin Oscar has the concept XYZ. Putnam has since expressed agreement with Burge's interpretation of the thought experiment. (See Putnam's introduction in Pessin and Goldberg 1996, xxi.)
A number of philosophers have argued that "water" for both Oscar and Twin Oscar refers to anything that is sufficiently water-like (i.e. the term's extension includes both H2O and XYZ). They reject, therefore, the contention that "water" is a rigid designator referring to H2O. John Searle, for example, argues (Intentionality: An Essay in the Philosophy of Mind) that, once we discover that our water is H2O, we have the choice of either redefining it as H2O (a classical reduction redefinition) or continuing to allow the term water to refer to anything with the basic properties of water (transparency, wetness, etc.). Searle suggests that in the Twin-Earth example, the second seems more plausible, since if Twin Earth doesn't have water, then all its water-based products will also be different. Twin ice cream, for example, will be constitutionally different, yet we will still be tempted to call it ice cream. Searle, along with others, considers this sufficient argument to "solve" the thought experiment altogether; others, such as Donald Davidson, feel that variations on the experiment can be used to draw some of the same conclusions.