Twin Earth

Twin Earth thought experiments originate with Hilary Putnam 1975. Twin Earth features in the context of arguments about INDIVIDUALISM. Individualism is the thesis that psychological properties are essentially intrinsic, like being made of gold, as opposed to being partly relational, like being a planet. Anti-individualism is the denial of individualism. Examples like the following often motivate anti-individualism. Suppose that in 1750 there was a planet, Twin Earth, exactly like Earth in all respects except that where Earth had H₂O, Twin Earth had a chemically different compound, XYZ. XYZ is like water in macroscopic respects. Nobody on Earth or Twin Earth could have detected the difference between them in 1750. But a modern-day chemist could distinguish them in her laboratory. Now we consider an Earth person, Oscar, and his identical twin on Twin Earth, T-Oscar. Oscar and T-Oscar are molecule-for-molecule replicas, identical in all intrinsic, physical respects. (Pretend that humans are not made out of water). Each twin uses "water" to apply to his local wet substance. And, let us suppose, each twin utters the words "Water is good for plants."

An anti-individualist might argue along the following lines. First, XYZ is not water. Chemistry informs us that water is H₂O, and, by hypothesis, XYZ is not H₂O. Second, Oscar's word "water" refers to water (H₂O) and nothing else: if Oscar had been confronted with a glass of XYZ and said "That's water," he would have been wrong. But T-Oscar's word "water" refers to XYZ and nothing else: if T-Oscar had been confronted with a glass of XYZ and said "That's water," he would have been right, because he would have been speaking Twin English. Third, when each twin utters "Water is good for plants," the utterance expresses the contents of his belief. Both twins understand their words and both speak sincerely, so what they say is precisely what they believe. But what the twins say is different: Oscar says that water is good for plants, and that is what he believes. What T-Oscar says and believes is something we might put as "T-water is good for plants," where "T-water" is a nontechnical word for XYZ.

Anti-individualists take the example to show that the contents of a person's beliefs are partly determined by his physical environment. Indeed, they are partly determined by factors that he does not know about, such as the underlying chemical constitution of water. Variant examples are designed to show that features of the social environment are also relevant. Here is one due to Tyler Burge (1979; adapted). Alf suffers from arthritis. One day, Alf wakes up with a pain in his thigh and comes to believe that his arthritis has spread. "Oh no! My arthritis has spread to my thigh," he mutters. He goes to his doctor who tells him that arthritis is, by definition, inflammation of the joints, and that therefore he cannot have arthritis in his thigh. Alf stands corrected and revises his belief.

We now turn to a new Twin Earth story. Again, Twin Earth is like Earth, barring one difference. Although the medical community on Earth applies "arthritis" only to inflammations of the joints, on Twin Earth they use it more generally, so that it would apply to the condition in Alf's thigh. Now T-Alf, Alf's twin, wakes up and mutters "Oh no! My arthritis has spread to my thigh."

Burge argues that while Alf believes he has arthritis in his thigh, T-Alf does not. T-Alf's word "arthritis" doesn't apply only to inflammations of the joints and hence does not express the concept of arthritis. Burge concludes that the contents of one's concepts are partly determined by the linguistic usage of those members of the community to whom one would (and should) defer. Recall that Alf accepts the doctor's assertion that by definition arthritis is a condition of the joints. It is in part because Alf is thus disposed to be corrected that his concept is a concept of arthritis and hence applies only to inflammations of the joints. T-Alf defers to different experts who use "arthritis" differently, and so has a different concept. This is so in spite of the exact similarity of the twins in all intrinsic respects.

Individualists have objected to the anti-individualist construal of the Twin Earth experiments and put forward counterarguments of their own. Responses to the Twin Earth examples have mainly been of two kinds.

The first kind of response concedes that, in the relevant cases, the twins' CONCEPTS refer to different things. For example Oscar's concept refers to water and T-Oscar's to T-water. But, it is argued, the essential psychological nature of a concept is not always given by what it refers to. Thus it is possible to hold that the twins' concepts are of exactly the same psychological type even though they refer to different things. This response needs to be supported by some method of individuating concepts that will generate the result that the twins' concepts are indeed of the same psychological type. (See Fodor 1987; Searle 1983; Loar 1988 for different attempts to do this.)

The second kind of response utilizes a distinction between commonsense intuition and science. It concedes that people have the intuition that the twins' concepts are different. But it holds that this is merely due to an unscientific conception of psychological states. Thus, while there is an intuition that, for example, Oscar's concept applies to H₂O but not to XYZ, and T-Oscar's to XYZ and not to H₂O, this intuition is wrong. In fact, both Oscars' concepts applied both to H₂O and to XYZ, and were identical even in respect of what they referred to. A scientific psychology, not bound to preserve intuitions, would thus treat the twins as psychologically indistinguishable (Segal 1989, 1991; Crane 1991).

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-- Gabriel Segal

References

Burge, T. (1979). Individualism and the mental. In P. French, T. Uehling, and H. Wettstein, Eds., Midwest Studies in Philosophy 4. Minneapolis: University of Minnesota Press.

Crane, T. (1991). All the difference in the world. The Philosophical Quarterly 41:1-26.

Fodor, J. (1987). Psychosemantics: The Problem of Meaning in the Philosophy of Mind. Cambridge, MA: MIT Press.

Loar, B. (1988). Social content and psychological content. In Grimm and Merrill (1988).

Putnam, H. (1975). The meaning of "meaning." In K. Gunderson, Ed., Language, Mind and Knowledge: Minnesota Studies in the Philosophy of Science, 7. Minneapolis: University of Minnesota Press.

Searle, J. (1983). Intentionality: An Essay in the Philosophy of Mind. Cambridge: Cambridge University Press.

Segal, G. (1989). Seeing what is not there. The Philosophical Review 98:189-214.

Segal, G. (1991). Defence of a reasonable individualism. Mind 100:485-493.

Twin Earth

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