One student asked me if the universe is necessary, and I think it's a good question. There are numerous ways to approach the issue, but let's think about it logically, with the help of ancient and medieval writers.
At the outset, I would define "necessary" in this context as "must exist/cannot not exist/not dependent upon anything or anyone" and "possible" as "contingent/could be otherwise." This means that if the universe is possible, although it exists, it might not have existed and the universe might fail to exist one day; furthermore, contingency implies dependence and a possible universe would not be dependent on itself but would be dependent on something outside itself.
So, to answer my student's question, it seems possible to formulate a disjunctive syllogism which might strike at the heart of the issue. By the way, the student is a theist but wants to build stronger arguments in order to support his case for God's existence.
The disjunctive syllogism I have in mind is the following:
1) Either the universe depends on itself or the universe depends on something else. (p v q)
2) The universe does not depend on itself. (~p)
3) Therefore, the universe depends on something else. (q)
This argument is a disjunctive syllogism, so it's deductively valid. And there seems to be good reasons for believing that the universe does not depend on itself to continue existing; nor did the universe cause itself to begin existing. Theoretically, without God, it's possible that the universe could stop existing. Either way, the universe appears to be possible rather than necessary.
Anselm of Canterbury contributes to this discussion by distinguishing between kinds of entities that might exist ab alio (through another) versus a being that possibly exists a se (through itself). When we reflect upon objects in the universe, it seems that they exist ab alio, not a se: that includes we ourselves, trees, animals, grass, and stars. Each of these things appear to be possible/contingent, but what about the universe as a whole?
When these kinds of discussions arise, scholars often point to Thomas Aquinas' five ways for the existence of God: the third way proceeds "from possibility and necessity." Edward Feser argues that by "possibility," Aquinas means the inherently transitory or
"inherent metaphysical instability" of hylemorphic objects, that is, objects composed of matter and form (e.g., trees, rocks, humans, dogs, cats, tables, etc.). The third way seems to teach (among other things) that objects which come into being and go out of being (hylemorphic objects which have potential and actuality, form and matter) are possible (contingent), not necessary insofar as they depend on something else for their existence.
Aquinas' argument is a posteriori or rooted in experience since we commonly see things come into existence, then pass out of existence. Because his argument is a posteriori, it's probable but not certain, given the premises--Aquinas is aware of how much his argument can accomplish. At this point, theists don't have to prove in an apodictic sense that God is a necessary being while the universe is not; again, Aquinas is only trying to demonstrate the probability that the universe is possible/contingent but God is not.
[To be continued]