Let p be the proposition that the last thing asserted by Shakespeare in his life is not true. This is a perfectly good proposition, and it is one that we can easily assert. Moreover, it is a proposition that Shakespeare could easily have asserted many times—but only if these times weren't the last moment of his life. Suppose in our world, w0, Shakespeare asserted p at time t1. (For all we know, he did!) Surely there is a world, w1, which is just like our world, but where Shakespeare is killed instantly after t1. In w1, then, Shakespeare did not assert p, since p is something that it is logically impossible for Shakespeare to assert as the last assertion of his life. But of course, in w1, Shakespeare uses the exact same words at t1 as he does in w0, and seemingly with the same intention.
So, what are we to make of this? On pain of contradiction, we must hold that in w1, Shakespeare fails to assert p in his last moment. If we think that words plus intention suffice to determine a proposition asserted (and even if we don't, we can perhaps stipulate a sense of "asserted" in which that is true, and make sure that that's the sense in p), it follows that what intentions one has can depend on what will happen later or that one's "words" include contextual features such as whether one dies shortly thereafter or when one utters them. In other words, we get a temporal externalism about intentions, or else a very weird notion of "words".
And we get an argument from this liar paradox against open futurism. For in w0 at t1, it is open whether Shakespeare will die right after t1 or not. But if he dies right after t1, he is not intending p. But it is true at t1 that he is intending p. Hence, it is true at t1 that he does not die after t1, which contradicts open futurism. Or, to put it differently, according to the open futurist, there is no fact at t1 as to what Shakespeare intends at t1.
Let's make the open futurist even more uncomfortable. Suppose at t1, you say:
- One day, I will open my mouth and utter a noise that does not express a true proposition.
- I just uttered a noise that did not express a true proposition