Is water H₂O? Is gold Au?

That water is H₂O is an established fact in chemistry. Yet in philosophy, this statement has prompted lots of debate and disagreements. By this I do not insinuate that philosophers question the scientific fact that water is composed of H₂O molecules. But the issues that they raise with respect to this statement are quite interesting and difficult to resolve.

Hilary Putnam formulated one of the most famous thought experiments in philosophy: the Twin Earth experiment. Assume that we live in a time (as we once did) when we are unaware of the elemental composition of water. Assume also that there is a world that is identical to ours in all ways but one. The liquid that looks, smells, and is used as we use water on earth – and which is also called water there – is made of a compound XYZ instead of H₂O, unbeknown to all. Should we say that on both earths, we mean the same thing when we talk of water?