Reply to Ian on Assertion and Probability

In response to an earlier post of mine, Ian Spencer has continued to press me to clarify and defend an argument I make in a recent paper to the effect that unqualified claims about the future (i.e., ones which say that something "will" or "will not" happen) normally carry a high degree of "causal force" as a part of their semantic content. More specifically, I argue that the strict and unqualified predictive use of "will" and "will not" connotes that the event spoken of is inevitable or, in Arthur Prior's apt phrase, now-unpreventable. What Ian doubts is that this causal force is actually part of the semantic content of such claims:

... Yes, Sally should be interpreted to believe that A is probable when she predicts it, but that doesn't mean that when she asserts the proposition that it will happen that what she is asserting is ... that it is probable. In order to make a claim of any kind at all, whether about past or present or future, if I'm making a genuine assertion and what I'm asserting is by my lights assertible, I will take it that what I am asserting is probable - but in no case does that necessarily mean that what I am asserting is that something is probable. ...

When I try to formalize your argument, I get something like this:
1. If X ... asserts that it will be the case that p then X believes that p is probable.
2. If (1) then (if X ... asserts that it will be the case that p then ... X is ... asserting is that p is probable)
3. So if X ... asserts that it will be the case that p then ... X is [asserting] that p is probable.

But why think (2) is true?

This is a fair question. Below is my answer. Note that instead of using 'p' to denote a putative future event, as Ian does, I prefer to reserve 'p' for the proposition asserted and denote the putative future event with an 'e' instead.

1. If X asserts the proposition p="Event e will occur at time t (=tomorrow)", then X posits a future-tensed state-of-affairs, viz., its being the case today that e be going to occur tomorrow.
2. A proposition is true at a given time iff the state-of-affairs that would be posited were p asserted obtains at that time. (E.g., "I am sitting" is now true iff my sitting obtains now.)
   
3. Hence, p is true iff the future-tensed state-of-affairs its being the case today that e be going to occur tomorrow obtains today. (1,2)
   
4. There is nothing that its being the case today that e be going to occur tomorrow could plausibly consist in other than present powers and dispositions.
5. These powers and dispositions suffice to make it true today that p iff they necessitate e's occurring tomorrow, i.e., they render e's occurring tomorrow now-unpreventable.
6. Hence, to assert p is to posit its being now-unpreventable that e occur tomorrow. (1,3,4,5)
   
7. To assert p one must believe p. In other words, one must believe that what one posits in asserting p is the case.
8. Hence, in asserting p, X believes that it is now-unpreventable that e occur tomorrow. (6,7)
   

Clarificatory note. Ordinary language in not normally very strict or precise. Often when we say "will happen" all we mean is "will probably happen". In (1) I take p to be asserted without hedging or qualification of any sort. In other words, "Event e will occur at time t" is to be interpreted strictly and literally. What my argument is intended to show is that when we are speaking strictly, "will happen" is equivalent to "will definitely happen".