"Practical" free will, as described by both of us:
> When people say that free will exists, they're saying that there is coherent notion of control over one's actions
> When humans say "free will", they are referring IMO in actuality to a real thing - a combination of many things, but basically agency + lack of prediction powers
You added a very specific phrase outside of the definition though, with no justification: "that grounds moral responsibility". That both implicitly implies moral responsibility, and also claims that it is grounded by this particular definition of free will. I'd be interested in how you got to those conclusions.
The "stricter/formal/whatever label you add here" version, that some try to argue for, including some compatibilists, goes more or less like this:
> Humans have the power to change the state/future of the universe and the universe is not "set" in any way
This feels a bit logically impossible to me, and it may also be something you don't agree with either, but it is still a definition that people use to mean free will, I have seen it way too often. However, the lack of this type of thing still has meaning is what I am trying to say. It is what leads to error theory for me, and then effectively what leads away from it. I wonder if that "away from" part might also solve some of your qualms with error theory.
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In terms of hard determinism, it seems to me you only listed off reasons that hard determinism doesn't sound easy, but made no argument against it itself. Just because an idea invalidates many ones that were previously popular does not make it any less true/false, nor does it prevent it from establishing new and helpful ideas on top of it, even if they do not really exist currently.
> you'll have to re-invent concepts of pseudo-moral blame
This is exactly where I think hard determinism has its value. There is no moral blame - hard determinism requires a morality without it, and that's okay. It simply is new because people have focused so hard on blame, but I think that "formal" definition of free will is useful to show precisely that blame is not in play.
> at which point you've just reinvented free will and called it something else
I do think much of this will happen, but the distinction between the definitions has consequences. I already argued about for blame, but I think there is much more to explore there.
> Moral error theory has problems of its own, so I don't think these all hang together very nicely.
Again, I'd love to hear the argument rather than a simple refutation, truly, in a non attacking or confrontational way. At this point I think the HN format is hurting more than it's helping, so if you're interested in talking more feel free to shoot me an email, @gmail.com. You seem well versed in this area and I've love to hear more details on your perspective and reasoning/principles.