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.
What, in general, is a subject matter? The answer is anything that somehow encodes the distinction between pairs of worlds that are just alike with respect to the subject matter in question and pairs that are not. A partition of the possible worlds would do, or equivalently an equivalence relation on worlds.
The present conception of aboutness and subject matters, following Lewis (1988), is intensional, not hyperintensional. It does not apply usefully to aboutness in mathematics or philosophy. The truth values of necessary and impossible propositions, regardless of whether they are expressed by sentences that speak of sines and cosines or by sentences that speak of the marital status of bachelors, turn out to supervene trivially on every subject matter.
Styrofoam is one kind of plastic. Therefore two worlds exactly alike with respect to plastic would a fortiori be exactly alike with respect to styrofoam. A proposition entirely about styrofoam is a fortiori entirely about plastic. The subject matter styrofoam is part of the more inclusive subject matter plastic. But plastic may in turn be part of other, still more inclusive, subject matters.
There is a most inclusive subject matter: being. Differences in being come in two sorts. There are differences in whether something is, and there are differences in how something is. Two worlds are alike with respect to being if they have no differences of either sort. Nothing exists in one but not in the other. Nothing has a fundamental property in one that it lacks in the other. No two (or more) things stand in a fundamental relation in one but not the other. And, since less-than-fundamental properties (and relations) supervene upon