A Necessary Being?

[Neo-Scholastic]

The only alternative to having a first member is an impossible infinite regress.

What makes an uncaused first member more possible than an infinite regress?

My initial response to the question was to consider it a schoolboy objection, but on second thought decided that you may actually be curious about why the demonstration of the 3rd Way excludes infinite regress as a viable option.

Keep in mind that proper interpretation of this demonstration only works for an essentially ordered series in which the existence of a thing depends on something essential to its existence. For example, human beings could not exist apart from some material composition. That is part of our essence. The existence of a human being depends, necessarily, on the existence of molecules and atoms that in turn depend on subatomic particles and so on down the line toward more fundamental levels of existence perhaps terminating in the quantum vacuum. But if there is an infinite regress then it's turtles all the way down below the quantum level. Seems absurd, but let’s just go with it anyway because lots of things appear absurd at the quantum level (at least in traditional modern mechanistic terms). So if we allow infinite regress then the chain of existence extends from human beings all the way down through an infinite series of turtles. Doesn’t that mean that at least one thing inside that infinite chain must exist?