is I will assume at first a fairly standard view of language, not my own weird view.
The following two claims are very plausible:
- Whether a particular sequence of words from a language L expresses a proposition does not depend on anything other than facts about L.
- A proposition is either true or false.
- The next line of words expresses a true proposition.
- The previous line of words does not express a true proposition,
Observe that the two lines of words can be written independently by two different people. Thus, whether a sequence of words uttered by me expresses a proposition can depend on what someone else says—even on what someone else says later, assuming (2).
We thus need to reject either (1) or (2) or both. In fact, I think we should reject (1). Rejecting (2) forces a non-classical logic. Call a sequence of words that does not express a proposition "nonsense". Then what we have learned is that whether a sequence of words is nonsense can depend on non-linguistic facts about the external world. Thus, just as we learned from Kripke that judging whether a proposition is possible is not in general a matter for an armchair investigator, so, too, judging whether a sequence of words is nonsense is not in general a matter for an armchair investigator.
Or at least that's what happens if one has a standard view of language. I myself have a non-standard one. On my view engaging in sentential anaphora (as in (3)) makes the anaphorically referred-to sentence be a part of one's own sentence—it is a way of taking up another's words and making them one's own. This is a version of deflationism. (By the way, I love the joke about deflationary semantics of "true". You want to be famous? You write a paper that says: "Everything Brandom says in his next paper is true." Then when Brandom publishes his paper, you say: "He's right, but I said it first.")
This all works a bit better on an eternalist theory of time.