Twin Earth thought experiment

A Twin Earth thought experiment was first posed by philosopher Hilary Putnam in 1975. Since then it has inspired a number of variations, which can be collectively referred to as Twin Earth thought experiments.

Putnam invites us to consider a planet, indentical to our own in every respect except that the place of water is occupied, not by H₂0, but by XYZ. The two "waters" are identical in every macroscopic property, although careful chemical analysis would reveal that they are, in fact, different chemically. Now of course there is, on this "twin Earth", a "twin Putnam", so suppose that both of them said something like "water is good for drinking". The argument can be made that when Putnam and twin-Putnam use the term "water", they are really referring to different things. Or are they?

The way one answers this question indicates whether one takes an Internalist or Externalist epistemological stance: an Externalist would argue that the two "waters" are different, while an Internalist would say that they are not.