... In 1976, my student Joseph LeDoux and I were studying a teenaged boy, known in the scientific literature as P.S. He had undergone split-brain surgery, which severed the pathways between the left and right hemispheres of his brain in an effort to curb his epilepsy. We were in the business of cataloging which mental functions were located in the left and right brains.

We had long known that language and speech processes were located in the left hemisphere, but absent from the right hemisphere, where various nonverbal skills are carried out. In normal, nonsplit brain cases, the two sides work together by integrating the information each receives. But in split-brain patients like P.S., that communication between hemispheres is cut off.

Here, visual perception plays an important role. Normally when a person focuses on a point in space, all visual information to the right of the fixated point is projected to the left brain and all visual information to the left of the point is projected to the right hemisphere. So by positioning images in certain places in relation to the patient’s focal point we can deliver visual information to either side of the brain. Images or words perceived by the left hemisphere can be easily described with language, but those perceived by the right cannot.

In the classic test for split-brain subjects, we flashed a picture of a snow scene to the right-half brain, while at the same time we flashed a picture of chicken claw to the left-speaking brain. When asked afterward what they saw, they could name only the claw. They knew nothing of the snow scene seen only by the disconnected right brain. The right brain does not speak.

We devised a slightly more interesting test for P.S. Instead of asking him what he saw, we presented him with two groups of picture cards and had him point to the one that best matched what he had seen. In quick fashion, his right hand (controlled by the left half of the brain) correctly pointed to a picture of a chicken, and the left hand (controlled by the right) correctly pointed to a snow shovel — meaning both sides of the brain had done its processing.

At this point in the test, the left brain actually didn’t see the snow scene and in fact should have no idea why the left hand pointed to the shovel. Of course, the left brain knew why the right hand pointed to the chicken, since all of the brain processes involved in such act were part of the left brain.

Then we stumbled on something very new. After 25 years of doing this test we finally asked a different question. “Why did you do that?”

It is the inventive interpretive mind first applying itself to our personal life and then to our social existence that is our core skill. Once humankind realized it possessed this technology, we seized on it to thrive in and control our niche on earth.

At that moment, his left brain was immediately confronted with a puzzle. Again, it knows why the right hand pointed to the chicken but why did the left hand point to the shovel? So, on the spot the left brain said, “Oh, the chicken claw goes with the chicken, and you need a shovel to clean out the chicken shed.”

That one simple observation, now repeated dozens of time on several patients, revealed another special capacity of the dominant left brain. We called this device the “interpreter” and have come to realize it is the storyteller, the system that builds our narrative and gives our many actions that pour out of us, frequently outside of our conscious control, a centrality, a story — our personal story. It is so powerful an addition to humankind that it masks the reality: We are, in fact, a confederation of relatively independent agents, each struggling to be part of our narrative that is our story. It turns out the left brain has another capacity potentially more important than language itself. The interpreter is the thing that sticks all of those parts together.

It has been almost 37 years since we first made these observations, which is about a microsecond in evolutionary time. Still, it has allowed me to see connections with other ideas about humankind. ...