> 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.

I always qualify my use of free will as being specific to the philosophical use in ethics because there's so much conflation around the term [1]. I hold that the term "free will" as used in various contexts are not necessarily the same, and that conflating them or trying to define a more general notion of free will that encompasses them all obscures more than it helps.

Per [2], I then conceive of free will in this context as "moral responsibility as accountability". I am not referring to free will as used in any other context, and I hope I've been strict about preserving that distinction.

> > Humans have the power to change the state/future of the universe and the universe is not "set" in any way

This definition compounds exactly the conflation that I think hurts progress in this debate. People often want more general abstractions to think about entire classes of problems rather than specific instances, but in this context, I think it obscures more than it clarifies.

> 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.

Compatibilism is necessarily compatible with determinism in which the universe is "set" in a specific way, that's the whole point. So I disagree with the definition. That said, just because it's "set" doesn't make it predictable, in which case it's outcome is not "set" in a way that can be known in advance.

So I would revise the clause "the universe is not 'set' in any way" to read, "the universe is not 'set' in a way that's relevant to responsibility", or something along those lines. But again, maybe you're thinking along more general lines that conflate the ethical properties with properties needed for other purposes (like an experimenters' freedom in science).

> 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.

I agree. I don't see anything in hard determinism which doesn't already have a satisfactory answer in Compatibilism though. The only quality which people seem to find appealing is a lack of blame, but as you later say, pseudo-blame will also likely be a feature of hard determinism in order to make it workable.

In the hundreds of debates I've had on this topic, pretty much the only reason people have used to justify hard determinism is that they don't think punishment is justified, and they see eliminating blame as a way to avoid punishment. This conflates justice with the simpler question of moral responsibility, and as a result, they erroneously believe that free will and Compatibilism necessarily entail punishment.

> Again, I'd love to hear the argument [refuting error theory] rather than a simple refutation, truly, in a non attacking or confrontational way.

Here's a rough summary of my thoughts when I first read it:

1. The Argument from Relativity: we're apparently not troubled at all by disagreement over natural facts (see: flat earthers), but disagreement over moral facts is somehow more problematic? We distinguish "fact" from "belief" for exactly this reason. I don't see any motivation for error theory here.

2. The Argument from Metaphysical Queerness, aka, moral properties are "qualities or relations of a very strange sort, utterly different from anything else in the universe", aka are moral prescriptions categorical or hypothetical imperatives: this appears to be the most subtle and misunderstood part of error theory, so maybe I'll delve in deeper at some point.

3. The Argument from Epistemic Queerness, aka, we would need "some special faculty of moral perception or intuition, utterly different from our ordinary ways of knowing everything else": the epistemic "strangeness" of moral properties partly follows from the strangeness of qualia. For instance, we have moral prescriptions around causing pain. I'm an eliminativist about qualia, so I think the apparent strangeness are cognitive tricks that we will unpack in time. The only other faculty needed is reason.

Bambrough addressed most of these [3] almost a decade before Mackie's paper on error theory. Bambrough's working example of a child undergoing surgery is precisely an example of a logical argument depending on qualia (epistemic queerness), to establish a moral fact. I'm sure Mackie would find that result very queer indeed!

Which isn't to say that error theory wasn't a valuable contribution, because it's absolutely true that most alleged moral prescriptions have been false. I simply don't think there's any conclusive reason to think they're all false, and per Bambrough, I think there are reasons to think we already know some moral facts.

[1] https://news.ycombinator.com/item?id=23480626

[2] https://plato.stanford.edu/entries/freewill/#FreeWillMoraRes...

[3] A Proof of the Objectivity of Morals - Bambrough (1969), https://www.reddit.com/r/philosophy/comments/3etl9b/a_proof_...