Twin Earth thought experiment

Twin Earth thought experiment

The Twin Earth thought experiment was presented by philosopher Hilary Putnam in his 1973 paper "Meaning and Reference" and subsequent 1975 paper "The Meaning of 'Meaning'", as an early argument for what has subsequently come to be known as semantic externalism. Since that time, philosophers have proposed a number of variations on this particular thought experiment, which can be collectively referred to as Twin Earth thought experiments. Edmund Husserl developed a similar thought experiment nearly 70 years earlier.

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 respects, which we refer to as ‘Twin Earth’. (We should also suppose that the relevant surroundings of Twin Earth are identical to those of 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, say Oscar, 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, or Toscar, and twin-earth water as twater.) "Ex hypothesi", their brains are molecule-for-molecule identical. Yet, at least according to Putnam, when Oscar says water, the term refers to H2O, whereas when Toscar 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 Toscar 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." (Putnam 1975/1985, p.227)

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 Toscar 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 Toscar 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.

Some philosophers believe that all such science-fiction thought experiments should be viewed with suspicion. They argue that when a thought experiment describes a state of affairs that is radically different from the actual one (or what we think it to be), our intuitions become unreliable, and significant philosophical conclusions cannot be drawn from them. Daniel Dennett, for example, calls Twin Earth and other experiments like it "intuition pumps", which play on a strong but ultimately illusory intuition.

Putnam, who is well known for changing his philosophical positions on a variety of issues, criticizes the experiment later in life because it is anti-functionalist.Fact|date=July 2007

An objection to the class of Twin-Earth style arguments for externalism can be raised in the form an argument that demonstrates that externalism (semantic or content) is incompatible with privileged self-knowledge. Here, privileged self-knowledge is simply the idea that one can know the content of one's thoughts without having to investigate the external world (for empirical evidence). Although, this type of argument does not disprove externalism alone, it is impressive because of the intuitive plausibility of privileged self-knowledge. Indeed, if one accepts this type of argument one is left with the option of choosing to accept either the truth of externalism or the truth of privileged self-knowledge (but not both).Fact|date=June 2008

ee also

* Swamp man
* Internalism and Externalism

External links

* [ Field Guide to the Philosophy of Mind article]


*Putnam, H. (1975/1985) [ "The meaning of 'meaning'"] . "In Philosophical Papers, Vol. 2: Mind, Language and Reality". Cambridge University Press.
*Putnam, H. (1973). "Meaning and Reference," Journal of Philosophy 70, 699-711.
*Dagfinn Føllesdal. (2001) Bolzano, Frege, and Husserl on Reference and Object. In "Future Pasts: The Analytic Tradition in Twentieth Century Philosophy". ed. Floyd, J., Shieh, Sanford. pp. 67-81(15). []
*Pessin, Andrew and Sanford Goldberg, eds. (1996) "The Twin Earth Chronicles: Twenty Years of Reflection on Hilary Putnam's "The Meaning of Meaning"." M. E. Sharpe,

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