Richard/Salmon against Kaplan

David Kaplan has this view that a proposition can be true at one time and false at another. So the proposition expressed by "I am cold" when I say that was true this morning when I was waiting for the bus, but now it's false because I'm inside and warm. But others, like Nathan Salmon and Mark Richard, think that propositions contain the time of utterance, so the proposition I expressed this morning is different than the one I express now.

Here's their argument against Kaplan:

If Kaplan were right, the following argument would be valid:

P. In 1971, Mary believed that Nixon was president and today she still believes that.

C. Today, Mary believes that Nixon is president.

We can see why Kaplan would be committed to the validity of it by seeing the form of the sentences when made explicit with their temporal operators:

P'. [In 1971] Believes (Mary, {Nixon, being president}) and [Today] Believes (Mary, {Nixon, being president})

C'. [Today] Believes (Mary, {Nixon, being president})

So it's a simple 'and-elimination' from P to C (or P' to C'). But, the problem is that this argument is not intuitively valid, if we think about the English formulations of P and C. The way to read P is that back in 1971, Mary believed then that Nixon was president, and the belief that she has now is that in 1971 Nixon was president. It doesn't follow that she thinks Nixon is now president - just that she believes he used to be.

From this, Richard and Salmon think it follows that propositions must contain times in them. I can't say I'm completely convinced by this, but I think it's understandable enough.