twin earth - Dictionary of Philosophy of Mind
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).
cf. Hilary Putnam, "The meaning of 'meaning'," Philosophical Papers, Vol. 2: Mind, Language and Reality, Cambridge University Press, 1975/1985.