I can't state exactly what I mean by "free will", any more than I can state exactly what I mean by "consciousness". No one has come up with a reductionist account of either. But since I actually do believe in consciousness, I can't dismiss free will as nonsense.

A clarification added in response to the instantaneous orgy of downvotes: I realize that Eliezer has provided a reductionist explanation for how he thinks "free will" should be interpreted, and for why people believe in it. That is not what I mean. I mean that no one has come up with a reductionist account for how what people actually mean by "free will" could work in the physical world. Just as no one has come up with a reductionist account for how what people mean by "consciousness" could work in the physical world.

If you find a reason to disagree with this, it means that you have a tremendously important insight, and should probably write a little comment to share your revelation with us on a reductionist implementation of naive free will, or consciousness.

in our own timeless and deterministic (though branching) universe.

That's the part I don't buy. I'm not saying it's false, but I don't see any good reason to think it's true. (I think I read the posts where you explained why you believe it, but I might have missed some.)