This question arose from the discussion of my recent question, [When is the first appearance of Phosphorus after March 21, 2021?] One of the other arguments in Kripke's 'Naming and Necessity' is that water is necessarily H2O, as 'water' and 'H2O' are rigid designators of the same thing.
Obviously (I think!), this argument must work equivalently for 'ice is necessarily H2O' and 'steam is necessarily H2O', and thus (perhaps less obviously, from the transitivity of identity) 'steam is necessarily ice'.
With regard to my earlier question, it was suggested that there is merely some ambiguity in ordinary usage, but I do not see any such ambiguity, either there or here: when someone says 'steam', we would not ordinarily wonder if they meant 'ice', and vice-versa. On the other hand, if they are metaphysically necessarily identical, why would it even be possible to be confused if someone freely mixed up the use of 'ice' and 'steam'? There may be a clue to resolving this in Frege's second puzzle, but it is not clear to me that the specific resolution there, in terms of the usage being in the context of propositional attitudes, generalizes to all uses of 'steam' and 'ice'.
So, if steam is necessarily ice, how does one resolve the apparent paradox of identity? And if not, where did I go wrong in concluding that it is?
The responses that I have received so far show that I should have been clearer in phrasing the question, which is not to argue for a seemingly nonsensical position, but is about what seems to me to be a paradox arising from a pair of Kripke's examples of a posteriori necessity: if you substitute "steam" for "Hesperus", "ice" for "Phosphorus", and "H2O" for "Venus" in Kripke's "Hesperus is necessarily Phosphorus" argument, you get an argument for the conclusion "steam is necessarily ice".
One may, of course, challenge this substitution, but I think there are grounds for accepting it. Another of Kripke's examples is that water is necessarily H2O, from "water" being a rigid designator of "H2O". It seems to me that both "steam" and "ice" are equally rigid designators of "H2O". They apparently fit the SEP definition: "A rigid designator designates the same object in all possible worlds in which that object exists and never designates anything else" - disambiguating, if someone feels the need, by saying "water ice" and "water steam" - or, at least, they fit the definition as least as well as do Hesperus and Phosphorus with regard to rigidly designating Venus; ice is not just H2O, but neither is Hesperus just Venus, and H2O is not always ice, but neither is Venus always Hesperus, and the above definition of "rigid designator" does not require them to be so, anyway. In neither the Hesperus / Venus case nor the ice / water case do we have a simple synonym, such as in another of Kripke's examples, Cicero / Tully.