 The “Twin Earth” scenarios introduced by Putnam (1975) are often invoked to support an externalist individuation of beliefs about natural kinds such as water, gold, or tigers. Twin Earth, as Putnam presents it, is a (hypothetical) planet on which things look, taste, smell and feel exactly the way they do on Earth, but which have different underlying microscopic structures; for example, the stuff that fills the streams and comes out of the faucets, though it looks and tastes like water, has molecular structure XYZ rather than H2O. Many theorists find it intuitive to think that we thereby mean something different by our term ‘water’ than our Twin Earth counterparts mean by theirs, and thus that the beliefs we describe as beliefs about water are different from those that our Twin Earth counterparts would describe in the same way. Similar conclusions, they contend, can be drawn for all cases of belief (and other intentional states) regarding natural kinds.