He sounds so authoritative. So sure. So truthful.

“It is estimated human beings only use 10 percent of the brain’s capacity,” says Morgan Freeman’s character, Professor Norman, in “Lucy.”

Let’s just get this out of the way — and quickly: whoever estimated this was wrong.

Not sorta wrong. Not kinda wrong. Not wrongish. Just straight-up, without equivocation, completely and utterly full of it.

The idea that humans use some minuscule percentage of our brains — three percent, five percent, 10 percent — is one of those myths that just won’t die. Now it gets new life thanks to the movie “Lucy” starring Scarlett Johansson as the title character and Freeman as the over-credentialed smart person telling her — and us — the reason the once-normal Lucy is able to kick butt and take names is because some drug made her a super-powered brainiac.

Is it a coincidence that Lucy, pioneering smartypants savior of the human race, shares a name with Lucy the Australopithecus afarensis, whose discovery in 1974 was one of the most significant breakthroughs in evolutionary science?

Well, maybe. Probably not. But maybe.

But truly, the line which serves as the premise for the plot of the movie is just a myth — one the Web site How Stuff Works calls “total nonsense.” A neurologist at Johns Hopkins School of Medicine in Baltimore told Scientific American it was laughable. We use an astonishing amount of brainpower just to correctly brew a cup of coffee.

“In walking toward the coffeepot, reaching for it, pouring the brew into the mug, even leaving extra room for cream, the occipital and parietal lobes, motor sensory and sensory motor cortices, basal ganglia, cerebellum and frontal lobes all activate,” Scientific American tells us. “A lightning storm of neuronal activity occurs almost across the entire brain in the time span of a few seconds.”

Not only do we use our entire brain, it commands a disproportionate level of energy: Our brains comprise three to five percent of our body weight but use 20 percent of the oxygen and glucose we take in.

So why did this notion persist even before we had the dulcet tones of Freeman’s voice drumming it into our heads? Well, because it makes us feel better about ourselves. How Stuff Works cited an explanation from neuroscientists Sergio Della Sala and Barry L. Beyerstein:

Despite much contrary data and its affront to logic, this hoary myth refuses to die, no doubt because of (you guessed it) the considerable uplift and encouragement it affords, not to mention the profit it generates for those who hawk self-improvement products that exploit the myth. If 90 percent of the brain were really a cerebral spare tire, as many of these hucksters claim, learning to tap its unused capacity would be the route to fabulous achievement, riches and fame — even, according to many New Age entrepreneurs, the pathway to psychic powers and transcendent bliss.

True to form, the film goes with psychic powers –“Imagine if we could access 100 percent and to see things begin to happen,” Professor Norman asks at a Serious Smart People gathering — and then kicks things up a notch by turning Lucy into a sorceress who can move people with just a flick of her wrist and whose eye color seemingly shifts with every advance in her IQ.

Of course, making us buy into things that are scientifically implausible is Hollywood’s bread and butter, and unlike Neil deGrasse Everything-Wrong-With- “Gravity” Tyson, most of us can usually suspend our disbelief. “Lucy’s” premise might just be a tad too facile considering it basically requires that one A) be literate and B) have access to Google in order to disprove it, but thankfully, we’ve got Adam Savage and Jamie Hyneman to sort out the trickier stuff.

For more information, here’s a Ted.Ed video about the 10 percent myth:

And the trailer for “Lucy”: