[Alamo-Girl]

Thank you so much for your post and your question!

So why then does the possibility that we might (very hypothetically) be able to predict human choices from our knowledge of physics and chemistry, mean that free will doesn't exist?

Because, under the metaphysical naturalist worldview, the physical realm is all that there is, that which exists (or will exist) on a timeline is the sum effect of cause, and there was a beginning. Moreover, under that worldview, the mind is what the brain does.

[Alamo-Girl]

Rats, I meant to use your voting probability as an example and forgot to do so in my haste to reply.

In the metaphysical naturalist scenario, your voting choice is the sum effect of the causes accrued over your life (timeline) to date. Thus, the parents you've had, your experiences in life (which were also effects of prior cause) - working in consort with your genetic code determine your vote.

IOW, you could be a robot or strong AI program running. How would you know you weren't?

[js1138]

Because, under the metaphysical naturalist worldview, the physical realm is all that there is...

This is an empty statement, because for the naturalist, "the physical realm" is anything that can be studied, and the list of things that can be studied grows almost exponentially with advancing knowledge. There is no final list of the contents of the "physical reality" set.

[js1138]

More to the point, if something from outside our known dimensions is impinging on our existence or our perception, any demonstration of this phenomenon will automatically be added to the box called physically reality. Anything you might wish to classify as supernatural or spiritual automatically qualifies as physical if it can interact with the known physical world.

The defining quality of physicality is not the nature of its form or structure, but whether it can be studied.