With the help of Mickey, from A Philosopher's Blather, I've been able to improve one of the modal cosmological arguments I've been working on.
1. Every existing being (thing, entity) is either contingent or necessary.
(1) is true by definition.
2. There is a possible state of affairs in which nothing contingent exists. (Premise)
"State of affairs" refers to the instantiation or non-instantiation of some concrete object or objects. Excluded are abstract objects, like numbers and propositions. There are many apples, for example, but it is possible for there to be no apples.
3. It is necessarily the case that possible states of affairs are possibly caused.
Assume that there is no cause of the non-existence of apples. Even granting this scenario, there is still a possible world in which the non-existence of apples is caused. Applied to the argument, then, there is a possible world in which the non-existence of contingent things is caused.
In fact, we may weaken the premise even further. Instead of being a sufficient explanation, suppose the cause in question is merely a partial explanation, such that:
3*. It is necessarily possible that possible states of affairs are at least partially caused. (Premise)
For instance, oxygen is a partial cause of water. Oxygen alone is not sufficient to cause water, but it is something needed.
4. It is possible for the non-existence of contingent beings to have at least a partial cause. (From 2 and 3)
5. Therefore, a necessary being exists. (From 1, 4 and S5)