Assertability and meaning

So I got into an interesting discussion with Alan Rhoda about the relation between rational assertability and meaning. He suggested that:
Whatever anyone must believe in order rationally to assert a proposition p is part of the meaning of p.
and used this assumption to argue for the the Peircean semantics for the future tense (on this view, the future tense is intrinsically modal, and to assert that an event E "will" happen implies that the world is now tending strongly (probability > 0.5) toward E's happening.).

I begged to differ. More details here.

The discussion continues here.