[Question] What is an agent in reductionist materialism?

I see a circularity problem in how folk talk about “agents”. I doubt I’m the first to notice this problem. So I wonder what the standard reductionist materialist answer is.

The puzzle is in the title.

To add a few more words:

I have no problem with things like “rock” even though there are no rocks in the absolute universe. We’re using many layers of abstraction. We can in principle break down (even literally!) what a rock is and go as far to the root of quantum math whatever as we like.

(It’s unclear to me what the floor is that we reduce things to in reductionist materialism. But I’m okay handwaving that for now by saying “something something math-and-physics something something”.)

Something like “The Odyssey” is trickier, but not too much so. It just requires that we add in some stuff about how brains work. Nothing too mysterious there. Unknown, sure, but not fundamentally mysterious.

But this seems to get very weird once you start talking about agents.

At first blush it looks the same as “The Odyssey”. An agent is just an abstraction, right? Implemented by some unknown but fundamentally non-mysterious process in a brain.

But this is circular. An abstraction for whom? What even is an abstraction, when you’re in the process of defining an agent? Is there some agent-free definition of an abstraction implicitly being invoked here?

I keep seeing people bump into this circularity when talking about AGI as an agent, and what alignment is. It’s like folks’ native intuitions assume that both quarks and agents are ontologically fundamental, but when pushed on this point they insist that really only the quarks are real… without anything that looks even vaguely to me like a justification for how you’d construct an agent out of quarks, or what that would mean.

In most spaces I’d assume this is because people just didn’t finish thinking this through.

I’m guessing that someone somewhere has noticed this and has given this careful thought. And maybe it’s even part of the core LW philosophy and I somehow missed it.

So I’m asking:

What is an agent in reductionist materialism?