This is the key thing, "first explicitly defining free will". Something I studied for a few years (full time).

Imagine this, a chess computer. Feed it more and more energy (time or computing power) and then it does better and better chess decisions. Give it infinite time or computing power. Does it have free will? No.

Because it has no insight into how or why it is doing what it is doing. It has no insight into how it made the decision it made at the 1 hour mark, and it has no insight into how it made the decision it made at the 2 hour mark if you let it keep thinking it over. It is thinking at introspectral magnitude zero, it has zero insight into its own brain.

Human brains are just the same, or any brains, only evolution is what programmed our brain's chess computer program, not a human programmer.

But if we had a brain scanner that allows us some insight into how exactly we made a decision, so that we can make a new decision knowing how we arrived at the previous introspectral magnitude zero decision, then we have an introspectral magnitude 1 decision.

Then we can use the brain-scan of the decision we made at introspectral magnitude 1 to find out how we made that decision, and make a new one (that either keeps the original decision, or doesn't), to get introspectral magnitude 2. Spectre 2 for short.

And we can keep going. If we keep going forever, with either an infinitely big brain that consumes an infinite amount of energy instantly, or an infinite amount of time, then we get up to an introspectrum level decision.

Introspectrum decisions is the closest thing to free will that exists in a causality-driven universe/multiverse.

This is kinda impractical for each decision, since infinite energy consumption for just ONE decision is rather impossible. But you can still approximate some introspectrum decisions within some degree of error that becomes negligible. A simple example is that you can always work out Pi to a suitable decimal count for whatever you are calculating, to such a degree of accuracy that you can't really decide that you're wildly wrong on the next trillion or infinite spectre levels. If you build a bridge with that level of Pi in your calculations, you're unlikely to later change your mind to any worthwhile degree. You might always find a better place to put the bridge, or a better bridge design, but you can approximate introspectrum level decisions in some situations.

For approximating introspectrum level decisions in humanity right now, you'd need to first be WELL versed in behavioral psychology (see Dan Ariely on youtube if this is the first time you hear that term), as well as evolution (see Selfish Gene by Richard Dawkins for a good intro), as well as applied statistics (don't really know any great popular science figure-heads for that, just find whoever tells you the average doesn't reflect the data pool. For example, if you get 60% more money and lose 50% for each coin toss, the average wealth will go up but most will end up bankrupt). If you are WELL versed in all these things (and probably reasonably versed in a few other things I can't fit in this character limit), then you CAN even approximate introspectrum decisions in some cases even without an actual brainscanner capable of determining exactly how you arrived at your decision.

I coined introspectrum type free will years ago, maybe I should make it easier to find on google.