@ACB,
ACB wrote:
'Water' has always meant H2O, but that meaning was only revealed when the chemical composition of water was discovered. Until then, it was hidden.
Or, to take an example from philosophy:
"If I am unmarried man, I am necessarily a bachelor" does not mean what most people think it does. It takes a philosopher to point out the modal fallacy and explain the true meaning of the word "necessarily" and the correct way to use it.
"Water" is the name of a natural kind. It isn't that "water" always meant H2O. Rather, it is that water always was H2O. And the meaning of "water" is catching up with our knowledge.
I really don't know that most people think, "If I am unmarried man, I am necessarily a bachelor" means. Most people do not make the distinction you make, since they have no occasion to make it. Anyway, it has nothing to do with the "true meaning of 'necessarily'" it has to do with the scope of "necessarily". (Just as the differnce between, "She sat next to the old men an women" and "she sat next to the old men, and women" has nothing to do with the meaning (true or false" of the word, "old", but everything to do with the scope of the word, "old" in each sentence.