Aquinas' Cosmological Argument

The Third Way-The Argument

1. Some things in nature are both created and destroyed.
2. Therefore, it is possible for these things to be and not to be. (1)
3. Everything which can fail to exist, does not exist at some point in time.
4. Therefore, it is impossible for these things always to exist. (2,3)
5. Therefore, if everything that is can possibly not exist, then at one time nothing existed. (4)
6. If something doesn't exist, it must be caused to exist by something else.
7. If at one time there was nothing in existence, then nothing would ever exist.
8. Therefore, if everything that is can possibly not exist, then nothing would ever exist. (5,6,7)
9. Therefore, there must be something which must always exist. (4,8)
10. Everything which is necessary is either necessary on its own, or because of something else.
11. There cannot be an infinite regress of things.
12. Therefore, there is a necessary thing which has its necessity on its own. (9,10,11)

Notes:

A necessary being is one which must always exist.

So, a proof that something isn't necessary is that, at some time, it doesn't exist. Hence, it is merely a possible or contingent being.

Perhaps the way to think about a being having its necessity caused by something else is to talk about the view that some (Descartes, for instance) believed that God caused the laws of logic and mathematics, even though these are thought to be necessary. [Of course, I worry that to make sense of this, either God is outside of time, or we have to start talking about possible worlds.]

Criticisms:

1. Why think that the Judeo-Christian God is the necessary being mentioned in the conclusion?
2. Why should we think that (3) in the argument is true?
   (Just because some possible/contingent things fail to exist at some point, it doesn't follow that all possible/contingent things fail to exist at some point.)
3. Does (5) really follow from (4)?
   (4') For every contingent being B there is a time t such that B does not exist at t.
   (5') There is a time t such that for every contingent being B, B fails to exist at t.
   (4*) For every person B there is a person A such that A is the mother of B.
   (5*) There is a person A such that for every person B, A is the mother of B.
   
4. Again, why should we think (11) is true? Perhaps it has intuitive force that explanation must have an end somewhere, but is that right? (And doesn't it beg the question against the agnostic/atheist?)