There has been much made of Hawking and Harris using brain scans to demonstrate a deterministic explanation of "free will". My question is, how do they treat a case where I think about moving my arm, but don't? How can the experiment they site test thoughts, subjective experience, etc. which do not lead to any outward physical effects? Must we accept that for all cases of mental phenomena that the brain scan test constitutes a proof?

Must we accept that for all cases of mental phenomena ... the brain scan test constitutes a proof?

I think the important philosophical question here is "A proof of what?" Suppose that science did somehow establish that all of our choices are causally determined by our earlier brain states. According to compatibilism, that result wouldn't threaten free will at all, and according to many compatibilists it would be good news indeed for free will. Before we get too hung up on whether brain scans are evidence in favor of determinism about human choices, let's ask the prior philosophical question "What difference would that make for free will?" You'll find this issue discussed many times on this website, including here:

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