Or “On the Non-Existence of Actual Alternate Possibilities”.
On my view there are three major categories of possibility: logical, metaphysical, and actual. Logical possibilities are those permitted by logic, metaphysical possibilities are those permitted by logic and [the laws of metaphysics, of which the best way we have come up with to discern is through conceivability experiments], and actual possibilities are those permitted by logic, [the laws of metaphysics], and actuality.
There are alternate logical possibilities all over the place. For example, there is no logical reason why there couldn’t be an object which is opaquely red all over and opaquely green all over in the same way at the same time. While such an object is inconceivable and so it is not metaphysically possible, there is nothing logically contradictory about it and so it is logically possible. But in addition to logical possibilities that are not metaphysically possible, all metaphysical possibilities are also logical possibilities.
There are also manifold alternate metaphysical possibilities. For example, there is no metaphysical reason why there couldn’t have been a larger or smaller quantity of matter in the universe. While the actual universe has exactly as much matter in it as it does and so there is no actual possibility that the universe has a larger or smaller quantity of matter, there is nothing inconceivable about it and so it is metaphysically possible. But in addition to metaphysical possibilities that are not actually possible, all actual possibilities are also metaphysical possibilities.
However, there are not any alternate actual possibilities. This is not to deny that there are any actual possibilities, but only that there are any alternate actual possibilities. This can best be understood by examining actual possibilities in three phases.
Firstly, there are actual possibilities concerning the past. Obviously it was actually possible for Barack Obama to get elected, because he did. But because he got elected, there is no longer any chance that in the actual world, Obama does not get elected. And this is true for all propositions concerning the past: there are no alternate actual possibilities.
Secondly, there are actual possibilities concerning the present. Obviously it is actually possible for it to be gloomy in Coeur d’Alene, because it is. But because it is currently gloomy, there is no chance it can actually be sunny. Therefore, there are not any actual alternate possibilities concerning the present.
Finally, there are actual possibilities concerning the future. Obviously what ever ends up happening in the future, such will have been actually possible. But because the future will unfold one way as a matter of fact and not another, there are not any actual alternate possibilities. Another way of putting this is that future contingents, those propositions concerning the future which are not true out of metaphysically necessity, each have a truth value. It is currently either true or false that Jesus will actually return at sometime in the future. And the fact that such is currently either true or false means that whichever one is the case is true of the actual world. And if it is true of the actual world, then no actual alternate possibility exists. The fact that it is conceivable that Jesus not return makes such scenario a metaphysical possibility, but not [necessarily] an actual one.