If in a deterministic universe, no observer B can 100% correctly predict the behavior of subject A, except when B is in the future of A, we can say that subject A has free will.
This strikes me as a very unintuitive definition of free will.
We often talk about free will experientially; defining it based on a specific external observer strikes me as odd. But before I critique that in any way, I’d love more clarification on what you mean by “observer.” Is this anything capable of prediction (e.g. a faithful simulation of the universe)?
But more importantly, I think “100% correctly” is doing the bulk of the work here. I fully agree with your claim that if we define free will in a manner similar to this, we will never reach it. But really, very little outside of statements within self-contained axiomatic systems can ever be held to the standard of 100% certainty. If your concept of free will hinges on the realistically minuscule chance that a random event will alter your decisions substantively, then I ask if this conception still resembles anything like the idea of “free will” as we tend to think of it.
Overall, I concede your claim follows from your definition. But I question the usefulness of such a definition in the first place. I think we can all agree that we cannot have 100% predictive certainty. The question is more whether or not we want to call that shred of uncertainty “free will.” Semantically, I think it’s confusing to call this “free will” when that is not usually the intended meaning of the phrase, but ultimately the decision is somewhat arbitrary as our experience remains the same regardless.
B (at least B as I intended him) is trying to create consistent general principles that minimize that inevitable repugnancy. I definitely agree that it is entirely impossible to get rid of it, but some take the attitude of “then I’ll have to accept some repugnancy to have a consistent system” rather than “I shall abandon consistency and maintain my intuition in those repugnant edge cases.”
Perhaps I wasn’t clear, but that was at least the distinction I intended to convey.