Some of this is a bit unreasonably sneerish toward St. Thomas, is what I think.
St. Thomas, in fact, does know that his ‘proof’ does not work given the assumption that the universe always existed.
I was once given the proofs of St. Thomas and when I noticed the fallacies, began bothering priests about it.
One day, I was introduced to a priest who supposedly had the answer, and he handed me a pamphlet containing quotes from St. Thomas where, to my surprise, St. Thomas admitted that the proof does not work if the universe is assumed to always have existed.
In other words, St. Thomas worked out his proofs with known assumptions, and re-worked them with others.
At the time his causality argument was popularized, the scientific understanding did not have Newton/Einstens etc. concepts.
Generally an object set in motion would be observerd to stop (today’s friction), or stop falling when it hit the ground (Teleological). The things in the sky were not understood yet, exactly.
It is not surprising that, due to these common experiences, his proof from causes would eventually become popular.
It is not fair to say that his arguments were fallacious in the context of the science of his day, even if the way of stating it assumed some common sense of his day which is now rejected.
Is it possible to show the neccesity of a first cause in terms of the logical science of today?
I think so.
The best of scientific advancement has led to concepts like density (which is clearly not infinite in the local sense in our real universe), conservation of matter, energy, and Entropy.
That third one is the universe killer.
No experiment, not even of Einstein’s caliber has ever shown the law of Entropy (third law of thermodynamics) to be incorrect.
In every chemical change, or physical change, and reaction, there is an irreversable component.
This law means that in any closed experiment, ever conducted, that any machine which does not receive energy from the outside will eventually cease to function (although it will still exist matter is not created or destroyed… ).
The energy will still be present, but unable to drive the machine any furthur.
No perpetual motion machines.
If the universe, which has a finite density, had been around forever it (as a machine) should have stopped functioning already.
Hence, it is illogical to say that the universe has been around forever.
The measurable density of the universe makes the size of the universe irrelevant. Make many small (closed) experiments. Have them running in parallel, all of them will stop.
Join them pair wise, they still stop.
Join all of the experiments to make a universe any size.
It will still stop.
The relitive uniformity, (finite density), of the universe makes the outcome certain.
So, there does (at least) have to be a point source of inexhaustable energy supply to posit that the universe has always existed, or one must speculate that the known laws of the universe change at some point to suit ones philosophy.
The former has not been found yet, and the latter is pure pride.
(Who knows, though, someone MIGHT discover an experiment…)
In any event, the universe does have a source of some kind, or the third law of thermodynamics is wrong (which is absurd).
And the source, logically, must have always existed…
St. Thomas was no Newton, but he didn’t do too badly.