twin earth - The duplicate planet in a series of thought experiments inspired by Hilary Putnam.
The twin earth thought experiments have been used by Putnam, Burge, and others to show the importance of our external environment to the individuation of mental contents. The position these experiments were constructed to support is called externalism. The two most famous of these experiments concern xyz (Putnam, 1975/1985) and tharthritis (Burge, 1979).
Putnam asks us to imagine a world, twin earth, where everything is identical to earth except the compound we call water (H2O) has a different atomic structure (xyz). XYZ is wet, colorless, odorless, and has all the same sorts of properties as H2O, nevertheless it has a different structure. Putnam claims that even though the residents of twin earth (twin earthlings) use the word 'water' in all the same ways earthlings do (because both are only familiar with macro-properties, let's say) the word 'water' on twin earth refers to xyz (or twater) and not H2O so the twin earthlings only have thoughts about twater. Clearly, since the word 'water' is used in all the same ways, and the twin earthlings are identical in every physiological respect to earthlings, the only way to determine the content of their thoughts is through reference to the environment they are in (the one that has twater).
Another way of looking at this result is to consider two physiologically identical individuals, one on twin earth and the other on earth. When the earthling thinks about 'water' s/he is thinking about H2O. When the twin earthling thinks about 'water' s/he is thinking about xyz. Since it is conceivable that these individuals are identical in every physiological (i.e. internal) respect the only way of determining what they are thinking about is through reference to their external environment. Thus, Putnam and Burge claim, we have to be externalists about our individuation of mental contents.