A Clarkean Cosmological Argument

The CCA, as we might call it, is inspired by the philosopher, Samuel Clarke, and is a blend of Leibnizian and Thomistic cosmological arguments:

1. Everything that exists has an explanation of its existence, either in the necessity of its own nature or in an external cause. (Premise, PSR)

2. Something presently exists. (Premise)

3. If something presently exists, then there was never a past time at which nothing existed. (Premise)

4. Hence, there was never a past time at which nothing existed. (From 2 and 3)

5. For every past time, something contingent has existed. (Assumption)

6. Hence, the series of contingent entities has an explanation of its existence. (From 1 and 5)

7. Nothing contingent can explain its own existence. (Premise)

8. Therefore, the series of contingent entities is explained by a necessary entity. (From 1, 6 and 7)

Of course, if the assumption of (5) is rejected, then we're brought to the existence of a necessary entity anyway, due to the truth of (4).  One might reject (6) on the grounds that the series of contingent entities isn't a thing, so-to-speak.  However, that objection is impertinent.  For, nothing contingent at all has to exist.  Why, then, does anything contingent exist, much less a whole series of contingent entities?  Its explanation can only be found in a necessary entity.