A Necessary Being?

[Gemini]

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.

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That's a rather uncharitable construal of the question. Instead of reading it as, "Why did Aquinas believe an infinite regress is impossible, given his understanding of causality?" it should be read as, "What argument against an infinite regress is potentially persuasive to skeptics?"

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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?

The entire infinite chain could exist as a brute fact, and not necessarily.