Or “On the Non-Actualizability of Non-Actual Metaphysical 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, and actual possibilities are those permitted by logic, the laws of metaphysics, and actuality.
As a consequence of my definitions, actual alternate possibilities do not exist. This is true simply because “actual” describes everything true of the actual world, and nothing more. If something is possible, but not “actual”, then it is not “actually” possible*, by definition. This is admittedly an unintuitive, specific, or technical use of the term “actual”, but it is how I’ve defined it here to facilitate more precise discussion.
Another consequence of my set of definitions is that non-actual metaphysical possibilities cannot be actualized, simply because they are not actual. Or more clearly:
Non-actualized possibilities are consequently not actual.
So we live in one world: the actual one.
*One might ask how something can be actually “possible”, without being “actually possible”. But keep in mind that on my view there are multiple types of possibility, of which “actual” is only one. In this way I am using “actual” to refer to propositions that concern the actual world, rather than as a synonym for “genuine”. In this way there can actually be “logical possibilities”, there can actually be “metaphysical possibilities”, and there can actually be “actual possibilities”. So if something is actually “possible”, but not “actually possible”, it must be either actually “metaphysically possible” or actually “logically possible”. But something cannot be “actually possible” and therefore actual, and non-actual at the same time, for such would be a contradiction.