The composition may change in terms of NaCl, etc., but if you do not have H2O then you do not have water. Your response?

I don't think I really understand the question here. — Metaphysician Undercover

Why am I not surprised?

Wouldn't we have to check every snowman, and make sure that it is not Frosty before we conclude that Frosty the Snowman does not exist. — Metaphysician Undercover

Yes Metaphysician, check every snowman in the whole world and double check that none of them is Frosty; that would be an excellent use of your reason.

This leaves "truth" as either completely arbitrary, or rescued from arbitrariness by subjectivity. — Metaphysician Undercover

The truth tables are important in Tarski’s First Order Logic. For example, the material implication truth table, whereby:

P.....Q.....if P then Q
==================
T.....T..........T
T.....F..........F
F.....T..........T
F.....F..........T

Kripke extended First Order Logic into Modal Logic K adding necessity and possibility, where the truth table shown above remains applicable to each accessible world.

Therefore truth, as expressed by truth tables, cannot be said to be arbitrary and is in this sense objective rather than subjective.

:up:

Yep. Truth tables for propositions and logical operators. Tarski also added satisfaction - f(a) is true IFF a satisfies f...

There's nothing arbitrary here. It's determined by the formal structure. The modal operators ◇ and ☐ are defined in relation to that formal structure by the introduction of possible worlds. The rules of logic and the structure of models fix truth independently of anyone’s opinion, so truth is objective in the formal sense.

Meta hasn't been able to follow this. But it is how it works.

Can I also at this stage express my appreciation to you, and @Frank for putting in the effort to understand what is happening here before launching into a critique. And thanks for the opportunity presented by this thread. paraphrasing is an excellent way to improve my comprehension.