“According to Harris free will is an illusion. Sure we can make decisions at any given moment but we don't know why we arrive at these decisions. This is both a mystery and definitive evidence that we are not free in the way that we think about free will in the everyday sense, in his view. And he presents scientific experiments to back up his findings along with thought experiments that could only be true if his extreme extrapolations of what the evidence indicates are correct (which is highly debatable).
This is a short book, but Harris could have tweeted this instead: "Man can do what he wills but he cannot will what he wills" ~ Schopenhauer (he does use this quote in the book) which is essentially what he is saying, but it would have been far more succinct, profound and far less contradictory than what he has come up with here. How can free will be a mystery and not a mystery (because courageous Sam Harris has all the answers) at the same time?
He dismisses hundreds of years of scholarship on the issue in favour of his irrefutable opinion about how human choice works. He presents us with a 'startling' fact: How is free-will possible in a cause-and-effect universe (wow so you've done philosophy 101)? Well, according to Harris it isn't and because he knows this it has helped him to become a more compassionate and moral being.
Aside from the self-congratulatory and self-assured tone of this essay, which is grating to say the least, I the reader can't help but think (because I am an automaton and have no say in the matter except to say what it is that I think) that many of his statements elevate ridiculousness to an art form. At one point in the book he dismisses the counterfactual 'evidence' that someone could have chosen this rather than that or that rather than this as something that cannot be substantiated. Fair enough. How can we know that we could have chosen otherwise (it's the illusion in action)? Then we are warned about the dangers of not considering Sam's amazing discoveries when it comes to policy making regarding violent crime, lest we become moral reprobates. Is this not a counterfactual? Extreme Fail. He chooses to be more moral now that he knows he's not free. WTF, another counterfactual and another instance of invoking choice when trying to disprove it. Hmm how does the cake taste? Doesn't morality at a certain level require free will anyway, because if we're all just doing what we're programmed to do than all actions should be just as valid as each other since they are inevitable.
Of course we're not totally free (and maybe not free at all). There are facticities seen and unseen to deal with, but even if free-will is an illusion (now I'm starting to make ridiculous statements myself) it is a useful one. At the experiential level this is how we seem to approach everyday life.
In the end, with everything we get to a point were we just have to accept a brute fact or axiom until new evidence comes along. What happened before the big bang? Why does Ockham's razor seem to work so well? What makes me choose to do what I do? For now it's just turtles all the way down with some of these issues. That doesn't mean we stop our search for knowledge in these areas, the answers are still out there, we (including Sam Harris) just haven't found them all yet.