Full Idea
It is surely the absence of experiences like the one Putnam describes that makes it reasonable to attach to molecular structure at least most of the importance that Putnam ascribes to it.
Gist of Idea
Presumably molecular structure seems important because we never have the Twin Earth experience
Source
John Dupré (The Disorder of Things [1993], 1)
Book Reference
Dupré,John: 'The Disorder of Things' [Harvard 1995], p.26
A Reaction
That is, whenever we experience water-like stuff, it always turns out to have the same molecular structure. Twin Earth is a nice thought experiment, except that XZY is virtually inconceivable.