It amazes me how modal intuitions were already quite sophisticated in the Middle Ages. I think we often give ourselves too much credit - or conversely, we don't give our predecessors enough credit - when it comes to philosophical and scientific advances. As early as the thirteenth century, Catholic philosopher, John Duns Scotus (so-named because he was a Scot) came up with this argument for a First Cause of essentially-ordered causes:
1. A First Cause is possible.
2. Whatever is possible is either contingent or necessary.
3. Whatever is contingent can be actualized.
4. A First Cause cannot be actualized.
5. Therefore, a First Cause exists necessarily.
The argument makes use of a weak version of the principle of sufficient reason (W-PSR). (1) should appear obvious to most of us. For, if a First Cause were not possible, then it would (by definition) be impossible. But, in order for something to be impossible, there must be a contradiction in the concept of it. The challenge is then for the opponent of the argument to demonstrate the impossibility of (1). I know of no argument that would even begin to suggest this.
(2) simply provides us with our available options. Since we have ruled out the First Cause's impossibility, the First Cause either exists in some but not all possible worlds (i.e. it is contingent), or it exists in all possible worlds (i.e. it is necessary).
(3), I think, is the crucial premise. The best way to support it is likely by connotation. We can simply provide examples of contingent things being actualized. For instance, it is possible that Planet-X should come into being at time-t. Yet, Planet-X (by hypothesis) does not exist in all possible worlds, even though it can be actualized in those worlds where it does not yet exist. Hence, we can think of Planet-X as a contingent entity.
Now, (4) ought to be granted upon reflection. If a First Cause were actualized, then it either actualizes itself, or else it is actualized by something causally prior to it. The First Cause obviously cannot be actualized by something causally prior to it; for then it would not be first in the series of efficiently-ordered causes. Nor can the First Cause actualize itself. In order for something to actualize itself, it would have to exist before causing its own existence, which is absurd.
(5) follows as a result of the truth of premises (1)-(4).