I still don’t understand why that isn’t an answer… why do we have to stop the chain and posit a first cause instead of saying the universe is eternal or something similar? Why exempt an entity from causality? I don’t need to show the whole chain; it’s enough to point out that there must be infinitely many stages and that we are simply at one of them.
As for the second question, I thought maybe there could be a reductio ad absurdum, and that way we could understand why it isn’t an explanation.

You can have an infinite regress and still know all the variables. For example, when there is a world that behaves cyclically. Like the big crunch. If so, the Rabbi’s proof falls.

That is just a declaration that there is an answer, but not an answer that you are actually presenting. By the same token, you could also say: there is an answer to this difficulty. Would you accept that as an answer? I asked you about “turtles all the way down.” Do you accept that as an answer? Why not?
Put differently: you can talk about infinity when you go backward from the origin of the axes. In such a process, you can point to every point on the axis and say at what time you will be there. But an explanation is supposed to begin at the beginning and not at the end, from the first stage (the beginning of the explanatory chain) to the result (= our universe). But to begin a chain at minus infinity and proceed to the origin is an undefined process. Therefore what you are proposing is a declaration, not an explanation or an answer.

I understand what you’re saying. What about circularity, like what Noam is suggesting here?

I didn’t understand the suggestion.

Everything the Rabbi said about it being impossible to give an explanation that has an infinite regress is only true when the explanation contains a chain with infinitely many factors. For example, turtle (1) lifted turtle (2), which lifted turtle (3), which lifted turtle (4) … which lifted turtle (n), and so on endlessly…
Therefore we can never explain it in reverse, from the initial turtle to the last turtle. Because we can never find the turtle that is at infinity either. It is undefined.
But when the explanation involves a defined number of unknowns — for example, we have only 2 factors, so that the entire process moves between them — then we know all the factors in the chain, and this is a valid explanation.
A concrete analogy for this: energy expanded and created matter (which is a conversion of energy), and that matter collapsed back into energy, which expanded into matter that collapsed back into energy… and thus the world we know was created within yet another one of those cycles of energy turning into matter. Something like the big crunch theory: this is how it always was and how it will continue to be.
In that kind of explanation, there is no reason it should count as a failed explanation. After all, we know all the factors in the above closed system.

I never understood this explanation.
(True, it’s a cliché, but) Saadia Gaon already asked: if time always existed, how did we get to the present?
You are claiming that an infinite amount of time has passed. How did we get out of it?

That’s Zeno’s paradox.

Noam, are you sure you understood what you yourself wrote? In any case, I didn’t.
A circular explanation is not an explanation, and the example you gave demonstrates that very well. The chicken laid the egg, and it produced the chicken, and so on in a loop. Does that explain anything? And through this you can also understand why an infinite linear explanation is likewise not an explanation (it’s just that the circle closes at infinity).

Yisrael, that’s not Zeno’s paradox. (A solution to Zeno’s paradox: time is made up of discrete parts. Even if that isn’t true, it solves the paradox but not Saadia Gaon’s question — meaning it’s something else.)

Zeno’s paradox assumes that time is like the real numbers and argues that you can’t traverse all the numbers up to a certain number and thereby reach it. My question works even if we assume time is made up of 1, 2, 3, etc. But Noam is suggesting that time began at minus infinity and slowly progressed until us. Which is of course impossible.

A correct distinction. But at a deeper level, the relation of the natural numbers to an infinite interval is equal to the relation of the real numbers to a closed interval. Examine this carefully.