It’s an old debate. It’s a conversation spanning millennia. It won’t go away.

Free will—do we have it or not?

The consensus tilts this way, then that.

But today, in the hot field of neuroscience, the trendy view, Michael Gazzaniga writes, is the “bleak view.”  Everything we do, think, feel, say, or fail to do is determined by our neural circuitry. The brain reigns supreme, tugging the mind along in its wake.

“The underlying contention,” Gazzaniga writes, “is that free will is just happy talk.”

In his new book Who’s in Charge? , Gazzaniga advances a counter argument.

Yet to do this, he proceeds along a surprising route. He devotes the first half of his book to laying out the massive collection of neuroscientific evidence showing that we have absolutely no idea what’s going on in our own brains—let alone control it.

And no one is better positioned than Gazzaniga to speak to this point. He is a veteran neuroscientist best known for his work with split-brain patients—people in whom the structure connecting the left and right hemisphere of the brain has been severed. Typically, this is a surgical procedure performed on people with severe epilepsy. It’s a last-resort technique to cut the corpus callosum, the bridge that joins up the two halves of the brain, in order to prevent seizure activity from spreading between them.

In his new book, Gazzaniga gives us an abridged account of the research he’s done on these patients. What’s fascinating about them is that, in many respects, they remain perfectly normal, their intellect, memory, and language abilities intact.

But Gazzaniga’s first great breakthrough was in discovering the places where this left/right disconnect is wildly apparent.

In one of the very first experiments of his career, Gazzaniga tested a split-brain patient named WJ. First he held up a picture of a spoon in such a way that it was only registered by the left half of WJ’s brain—the half that specializes in language. When asked if he had seen anything, WJ responded normally, “a spoon.” But when the same picture was held up so that only WJ’s right brain registered its presence, WJ reported that he hadn’t seen anything at all.

This was the crucial moment that led Gazzaniga deep into the depths of split-brain patient research. What he found was that, once severed, each half of the brain has no idea what’s going on in the other. WJ’s vision wasn’t impaired since the reason he made no mention of a spoon when it was displayed only to his right brain was that his left brain, with its language centers, had no idea it was there.

Years of this kind of research led Gazzaniga to be able to characterize the differences between the left brain and the right brain. The right, he says, “lives a literal life.”  It doesn’t extrapolate, it doesn’t narrate, it doesn’t generalize. It registers in an exact, concrete fashion what’s going on around it.

The left hemisphere plays a different role. It’s our resident storyteller.  “The left hemisphere was the intellectual,” Gazzaniga discovered. It is our brain’s  “interpreter.”

It’s the left brain that spins a narrative out of all the disconnected bits of information swimming up into our conscious view.  The funny thing, however, is that the stories the left brain produces are largely if not entirely wrong.

“The reality is, listening to people’s explanations of their actions is interesting—and in the case of politicians, entertaining—but often a waste of time,” he writes.

The reason for this is that the left brain works with whatever becomes conscious, but consciousness is the ultimate slow poke. It lags behind. It’s walking while the non-conscious brain is sprinting to the finish line, processing what’s happening around us, making a decision about how to respond, even beginning to execute that response. Our conscious awareness is the last to find out.

So what “the interpreter” narrates is necessarily after the fact. Whatever we find ourselves doing, our interpreter will cobble together some reasonable explanation, but it is always a retrospective account.

Besides his own research, Gazzaniga touches upon a huge swath of brain science backing up the basic point that we have no idea what’s going on, but we need to tell ourselves otherwise.

This is the illusion that humans daily entertain: we are the masters of our domain, aware of what we’re doing and why we’re doing it.