In logic, an eternal sentence is a proposition whose truth value is permanently fixed. The term is from Quine, who I think coined it. Eternal sentences come in two flavours, true and false, but there's no McDonald's Twist Cone action happening here – shit stays the same regardless of time, context or speaker.
True, but not eternal, sentences
- It is the twenty-first century.
- There is a smallish planet orbitting Sol whose inhabitants call it Earth.
True eternal sentences
False, but not eternal, sentences
- The bananas on my table are ripe. (Actually, right now they are still very green)
False eternal sentences
But language changes! Who says that 'water' is even a word three thousand years from now?
You have to imagine the sentences relativized to their language of utterance, even when that bit isn't exactly spelled out. Quine writes in The Philosophy of Logic (1970) that
eternal sentence that was true could become false because of some
semantic change occurring in the continuing evolution of our own
language. Here again we must view the discrepancy as a difference
between two languages: English as of one date and English as of
another. The string of sounds or characters in question is, and
remains, an eternal sentence in earlier English, and a true one; it
just happens to do double duty as a falsehood in another language,