All of the modal propositions I can think of are most reasonably analyzed as a modal operator applied to a proposition, and possibly other arguments. In the following examples, I'll write the arguments to the modal argument in parentheses:
- It is possible that (there are black swans).
- It has always been the case that (2+1=3).
- (Joe) believes that (all crows are black).
Note that the last one takes two arguments, an object (Joe) and a proposition (all crows are black). I can't think of any modal propositions that don't involve a modal operator that takes a proposition as one of the arguments, are there any? Are there any modal propositions that just look like a regular predicate applied to one or more regular, non-propositions objects?
Also, I imagine that by taking enough liberties, one could always express a modal proposition in this form, but are there any where it is natural to express the proposition as just a predicate applied to objects: p(x, y, z)?
What I mean by modal in this context is non-truth-functional. For example, if p(x,y,z) were a modal proposition, then some tautologies such as P->(Q->P) might not hold if P is p(x,y,z).