My intuition has always been that 'free will' isn't a binary thing; it's a relational measurement with a spectrum. And predictability is explicitly incompatible with it, in the same way that entropy measurements depend on how much predictive information you have about a system. (I suspect that 'entropy' and 'free will' are essentially identical terms, with the latter applying to systems that we want to anthropomorphize.)

No one has ever come up with such a criterion

There are a number of useful terms for which no one has ever come up with a precisely stated and clearly defensible criterion. Beautiful, good, conscious, etc. This surely does indicate that there's something unsatisfactory about those terms, but I don't think the right way to deal with it is to declare that nothing is beautiful, good, or conscious.

Having said which, I think I can give a not-too-hopeless criterion distinguishing agents we might reasonably want to say have free will from those we don't. X has free will in regard to action Y if and only if every good explanation for why X did Y goes via X's preference for Y or decision to do Y or something of the kind.

So, if you do something purely "on autopilot" without any actual wish to do it, that condition fails and you didn't do it freely; if you do it because a mad neuroscientist genius has reprogrammed your brain so that you would inevitably have done Y, we can go straight from that fact to your doing Y (but if she did it by making you want to do Y then arguably the best explanation still makes use of that fact, so this is a borderline case, which is exactly as it should be); if you do it because someone who is determined that you should do Y is threatening to torture your children to death if you don't, more or less the same considerations apply as for the mad neuroscientist genius (and again this is good, because it's a borderline case -- we might want to say that you have free will but aren't acting freely).

What does this criterion say about "normal" decisions, if your brain is in fact implemented on top of deterministic physics? Well, an analysis of the causes of your action would need to go via what happened in your brain when you made the decision; there would be an "explanation" that just follows the trajectories of the elementary particles involved (or something of the kind; depends on exactly what deterministic physics) but I claim that wouldn't be a good explanation -- in the same way as it wouldn't be a good explanation for why a computer chess player played the move it did just to analyse the particle trajectories, because doing so doesn't engage at all with the tree-searching and position-evaluating the computer did.

One unsatisfactory feature of this criterion is that it appeals to the notions of preference and decision, which aren't necessarily any easier to define clearly than "free will" itself. Would we want to say that that computer chess player had free will? After all, I've just observed that any good explanation of the move it played would have to go via the process of searching and evaluation it did. Well, I would actually say that a chess-playing computer does something rather like deciding, and I might even claim it has a little bit of free will! (Free will, like everything else, comes in degrees). Still, "clearly" not very much, so what's different? One thing that's different, though how different depends on details of the program in ways I don't like, is that there may be an explanation along the following lines. "It played the move it did because that move maximizes the merit of the position as measured by a 12-ply search with such-and-such a way of scoring the positions at the leaves of the search tree." It seems fair to say that that really is "why" the computer chose the move it did; this seems like just as good an explanation as one that gets into more details of the dynamics of the search process; but it appeals to a universal fact about the position and not to the actual process the computer went through.

You could (still assuming determinism) do something similar for the choices made by the human brain, but you'd get a much worse explanation -- because a human brain (unlike the computer) isn't just optimizing some fairly simply defined function. An explanation along these lines would end up amounting to a complete analysis of particle trajectories, or maybe something one level up from that (activation levels in some sophisticated neural-network model, perhaps) and wouldn't provide the sort of insight we seek from a good explanation.

In so far as your argument works, I think it also proves that the incompatibilists are wrong. I've never seen a really convincing incompatibilist definition of "free will" either. Certainly not one that's any less awful than the compatibilist one I gave above. It sounds as if you're proposing something like "not being reliably predictable", but surely that won't do; do you want to say a (quantum) random number generator has free will? Or a mechanical randomizing device that works by magnifying small differences and is therefore not reliably predictable from any actually-feasible observations even in a deterministic (say, Newtonian) universe?

I don't think you answered my question: what's the underlying principle?

I'm not actually sure.

I do think that there's really incredibly good evidence that the Adam and Eve story is not literal, that it's rather meant as a fable, to illustrate some important point. (It may be some sort of heavily mythological coating over an internal grain of historical truth, but if so, then it's pretty deeply buried).

I'm not entirely sure what that point is. Part of it may be "the rules are there for a reason, don't break them unless you're really sure". Part of it may be intended for children - "listen to your parents, they know better than you". (And yes, some parents are bad news; but, by and large, the advice "listen to your parents" is very good advice for toddlers, because most parents care about their toddlers).

And Adam and Eve as portrayed in the Genesis narrative don't seem to have been anything like as incapable as you were when you had no idea why scribbling in one book might be worse than scribbling in another.

I do wonder, though - how old were they supposed to be? It seems that they were created in adult bodies, and gifted from creation with the ability to speak, but they may well have had a toddler's naivete.

Would your parents have put toddler-you in a room with a big red button that launches the missiles, sternly told you not to push it, and then left you alone?

Not if they had any option.

If they had, what would you think of someone who said "oh, it's all CCC's fault that the world is a smoking ruin. He pushed that button even though his parents told him not to."?

Toddler-me would probably have expected that reaction. Current-me would consider putting toddler-me in that room to be horrendously irresponsible.

It certainly makes more sense that way than as history. But even so, it comes down to something like this: "Remember, kids! If you disobey your parents' arbitrary instructions, they're likely to throw you out of the house." Ah, the piercing moral insight of the holy scriptures.

I see it as more "obey your parents, or you're going to really hate what comes next". It's not perfect, but it's pretty broadly applicable.

on that day your death will become inevitable

That's an interpretation sometimes put on the text by people with a strong prior commitment to not letting the text have mistakes in it. But does what it says actually admit that interpretation?

If you know ten words of Hebrew, then you know ten more words of Hebrew than I do.

In short, I have no idea.

Doh! Thanks for the heads-up.

Not quite. It needs to have TWO labels. On the left it says, "DO NOT PRESS" and on the right it says "PRESS THIS BUTTON".

Hmmmm. Not sure that's quite right. The serpent wasn't an authority figure. Maybe label the button "DO NOT PRESS" and add a stranger (a door-to-door insurance salesman, perhaps) who claims that you'll never know what the button does until you try it?

Is it okay for a three-month-old baby, who does not understand what it is doing, to bite a kitten's tail?

No. Of course not. Why would you doubt it?

(And is it okay for the kitten to then claw at the baby?)

Yes. Of course. Why would you doubt it?

Okay, in both cases, the situation is basically the same - a juvenile member of one species attacks and damages a juvenile member of another species. Why do you think one is okay and the other one is not?

Delegation?

Huh??? Why would an omnipotent deity need to delegate?

Because it's really boring to have to keep trying to individually explain the same basic principles to each of a hundred thousand near-complete idiots?

Cain knew it was wrong to kill Abel

How do you know that? Just because he denied doing it? Maybe he thought it was perfectly OK to kill Abel, but wanted to avoid what he saw as unjust punishment.

If so, then he sought to avoid what it from every other person in the world (Genesis 4, end of verse 14: "anyone who finds me will kill me"). Either he thinks that everyone else is arbitrarily evil, or he thinks they'd have reason to want to kill him.

Also, let's look at man's next transgression:

"Ge6:5 And God saw that the wickedness of man was great in the earth, and that every imagination of the thoughts of his heart was only evil continually."

In other words, God's first genocide (the Flood) was quite literally for thought crimes. Does it seem likely to you that the people committing these (unspecified) thought crimes knew they were transgressing against God's will?

I'd always understood the Flood story as they weren't just thinking evil, but continually doing (unspecified) evil to the point where they weren't even considering doing non-evil stuff.

Okay, but we can still predict the output of the computer at any given, finite, time step.

Really? How exactly would you do that? Because the only way I know of to tell what a computer is going to do at step N once N is sufficiently large is to build a computer and run it for N steps.

Simulate the algorithm with pencil and paper, if all else fails. (Technically, you could consider that as using your brain as the computer and running the program, except you can interrupt it at any point and investigate the current state)

the coin tosses can be replaced by a person with free will calling out "head!" and "tail!" in whatever order he freely desires to do

I really don't get what point you're trying to make here. My position is that people do not have free will, only the illusion of free will. If it were possible to actually do this experiment, that would simply prove that my position is correct.

The point I'm trying to make with the coin/time-traveller example is that knowledge of the future - even perfect knowledge of the future - does not necessarily imply a perfectly deterministic universe.

See, this is exactly what I'm talking about. This is kind of like arguing over whether Shakespeare's plays were really written by Shakespeare, or by someone else who happened to have the same name. You've lost critical information here, namely, that there is a connection between GW-the-historical-person and GW-the-myth that goes far beyond that fact that they have the same name.

(Side note: I don't actually know GW-the-myth. It's a bit of cultural extelligence that I, as a non-American, haven't really been exposed to. I'm not certain whether it's important to this argument that I should)

Or take another example: Buzz Lighyear started out existing as as an idea in someone's head. At some later point in time, Buzz Lightyear began to exist also as a cartoon character. These are distinct because Buzz-as-cartoon-character has properties that Buzz-as-idea doesn't. For example, Buzz-as-cartoon-character has a voice. Buzz-as-idea doesn't.

Hmmm. An interesting point. A thing can certainly change category over time. An idea can become a character in a book can become a character in a film can become ten thousand separate, distinct ideas can become a thousand incompatible fanfics. At some point, the question of whether two things are the same must also become fuzzy, and non-binary.

Consider; I can create the idea of a character who is some strange mix of Han Solo and Luke Skywalker (perhaps, to mix in some Star Trek, they were merged in a transporter accident). It would not be true to say that this is the same character as Luke, but it would also not be true to say that it's entirely not the same character as Luke. Similarly with Han. But it would be true to say that Han is not the same character as Luke.

So whether two things are the same or not is, at the very least, a continuum.