Is there a source for the idea that anything not prohibited is permitted? Or is that simply the reasonable assumption?
It is well known that the hairsplitters have debated this… (an interesting example: Maharik, if I remember correctly, in root 139—about root 74)

This is the “principle of legality.” Why would I need a verse? It is simple logic! At the outset everything was permitted, and then a few things were prohibited to us. Whatever was not prohibited remained permitted. An original presumption.

I don’t understand—this itself is the discussion: what things were like at the outset. And the principle of legality certainly does not prove your point. Maybe the logic goes the other way: everything was prohibited, and then the Torah came and permitted certain parts. (And it doesn’t matter what form the Torah’s statement takes, whether prohibition or obligation.)

Meaning, before the Torah was given everything was prohibited? And now do you have a source permitting you to eat caraway bread? This is a waste of time to discuss this nonsense hairsplitting.

It is known that Noah or his descendants were permitted to eat animals. And from then on it was permitted. Where was it prohibited to them?

I hope this is not a continuation of the unnecessary discussion above.
Apparently from logic. Why are you taking something that is not yours? Especially making irreversible use of its body. And also in the Garden of Eden: “Of every tree of the garden you may surely eat”…