Regarding authority — the novel idea of the Kesef Mishneh is that the authority of the Talmud is founded on acceptance by the people from below. What is the source for that? That is, where do we find that acceptance from below creates binding formal authority?

See Beit Yishai there.

I don’t have the book. I’d appreciate it if the Rabbi could write me the basis and force of the idea, if possible. I was just thinking that maybe it’s simply a logical argument. That is, just as any collective group can take an oath about something and thereby obligate those who come after them. But according to that, logical reasoning is Torah-level. If so, that raises a difficulty for Nachmanides’ view that “and you shall do according to all that they instruct you” refers only to interpretation and not to legislation. There’s much to expand on here.

I sent it to you by email. Clearly that is the reasoning. I didn’t understand what the difficulty is according to Nachmanides.

What I meant to say is that if this is indeed the reasoning, and reasoning is Torah-level, then the foundation of “the rulings of the sages” may not come from the verse, but it is still a logical principle that stands on the level of Torah law. If so, why does he ask against Maimonides (who holds that “and you shall do according to all that they instruct you” includes both legislation and interpretation) that according to his view a rabbinic-level doubt should still be treated leniently, when it ought to be treated stringently? The question applies according to his own view as well!

To raise that difficulty, you didn’t need to get as far as acceptance by the many. In every rabbinic prohibition there is also some reasoning, because otherwise they wouldn’t enact it. So why don’t such prohibitions have the status of Torah law?
One can distinguish in several ways: (a) there are different levels of reasoning (strong and weak), and not every logical argument is Torah-level. (b) One can distinguish between interpretive reasoning and reasoning that creates a new law (I discussed this at length in my article on logical arguments; take it from there). (c) There is a difference between reasoning about the substance of the matter and reasoning about authority. The authority of the sages to legislate and decree is indeed Torah-level, but that does not mean that the content of the exposition or decree is Torah-level. I discussed this at length in my article on the first root (in the book Yishlach Sharashav, which is here on the site), and in the first section of my book Ruach HaMishpat.

Thank you very much for all the references. I’ll look into all of this.

[I’ll just say that regarding (a), it seems to me that the reasoning behind the sages’ creation of laws is a strong argument. So that distinction doesn’t really help.]

Correct. I wrote it only to complete the picture.