If somebody said that he hears voices in his head, or that he has a twin that no one can see, or believes in an invisible world, then you would infer that he is schizophrenic, or suffers some similar mental illness.
If somebody believes in Heaven, Hell, angels, and devils, and prays to an invisible god, you would infer that he is religious, but not crazy.
But what do you make of somebody with an advanced scientific degree who believes:
* You have identical twins in distant universes.
* Our world is just a simulation, running on a computer in a more advanced civilization.
* The universe constantly splits into parallel universes, where every possibility is played out for real.
* All events have been pre-determined, from the first second of the Big Bang.
* We have no free will, and all actions are controlled as with a robot or puppet.
All these things seem like symptoms of mental illness to me.
Another tipoff is having crazy ideas about randomness. Insane people sometimes think that nothing is random, and every coincidence is a manifestation of some bizarre conspiracy of causes. Or they think that randomness is one of the fundamental forces in the universe, disrupting everything. Either way, the insane man is troubled by imaginary demons that are sabotaging his life.
So how am I supposed to think about respected physics professors who believe in this crazy stuff? I do not have a good answer for this.
I have reviewed some debates on free will, from respected professors. Nearly all of them reject free will. I think that they are all insane. I wonder how they can even function in their daily lives with such peculiar beliefs.
Sam Harris is someone who seem intelligent, and even has a PhD in cognitive neuroscience, and yet vehemently argues that he is a biochemical puppet with no free will. In his case, it appears that his free will was destroyed by recreational psychodelic drugs that he began taking at age 18. In his podcasts, he talks about practicing Eastern Buddhist meditation, in order to try to get free will. He sounds very convincing, just as the schizophrenic can be very convincing that he really does hear voices in his head.