Another Modal Cosmological Argument

This one is inspired by Bl. John Duns Scotus:

1. Necessarily, everything that exists is either contingent or necessary. (Definition)

2. Possibly, every contingent thing has a cause. (Premise)

3. Necessarily, the regress of contingent causes is either finite or infinite. (Premise)

4. Possibly, the regress of contingent causes is finite. (Premise)

5. Necessarily, a necessary entity cannot have a cause. (Premise)

6. Possibly, a necessary first cause exists. (From 1, 4 and 5)

7. Therefore, a necessary first cause exists. (From 6 and S5)

I think the most difficult premise to defend is (5).  If we understand necessity as being self-explanatory, then (5) is most certainly true.  If, on the other hand, there can be more than one necessary entity, with one being dependent (but not contingent) on another, then the former wouldn't be classified as a first cause.

It's at this point that we could supplement the argument with additional reasons for the unicity of a necessary entity.  As St. Thomas Aquinas summarizes:

"[I]f there are two beings of which both are necessary beings, they must agree in the notion of the necessity of being.  Hence, they must be distinguished by something added either to one of them only, or to both.  This means that one or both of them must be composite.  Now . . . no composite being is through itself a necessary being.  It is impossible therefore that there be many beings of which each is a necessary being."  (Summa Contra Gentiles, ch. 42.)