ABSTRACT
Any proposition has a subject matter, on which its truth value supervenes. Suppose that a certain proposition is entirely about styrofoam. Then its truth value supervenes upon the totality of the world’s styrofoam. If two possible worlds were just alike with respect to their styrofoam – if they had styrofoam of just the same kind at just the same places and times – then, no matter how much those two worlds differed otherwise, the proposition would be true in both worlds or false in both. Conversely, if some proposition never differed in truth value between two worlds that were just alike with respect to their styrofoam, then that proposition would have to be entirely about styrofoam.
