A Twin Earth thought experiment was first posed by philosopher Hilary Putnam in 1975 in order to argue for an externalist, as opposed to an internalist, epistemological stance. (See Internalism and 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.
Putnam originally proposed the thought experiment in something like the following manner:
- Consider a planet, indentical to our own in every respect, except that the place of water is occupied, not by the chemical compound H20, but by another compound, XYZ. The two "waters" are identical in every macroscopic property (e.g., both are liquid at normal temperatures, both are good to drink, both are good for swimming in, both are clear when in pure form). Only careful chemical analysis would reveal that the two aren't the same at the molecular level. Now of course there is, on this "twin Earth," a "twin Putnam," so suppose that both of them come to tell their respective (twin) children something like, "water is good for drinking."
Now the question arises: when Earth-Putnam and twin-Putnam say "water," does that word refer to the same thing for each of them, or does it refer to a different thing in each case? According to Putnam (the real one, on our Earth, writing as philosopher), "water" refers to something different for each of them; for Earth-Putnam, it refers to H2O, while for twin-Putnam, it refers to XYZ.
Other people argue that is not the case, however.
Putnam, H. (1975/1985) The meaning of 'meaning'. In Philosophical Papers, Vol. 2: Mind, Language and Reality. Cambridge University Press.