What preserved Bagenholm’s memories and abilities, over hours, in a state of clinical death? Scientists believe that every thought, every sensation, is a set of tiny electrical impulses coursing through the brain’s interconnected neurons. But when a little girl learns a word, for example, her brain makes a record by altering the connections themselves. When she learns to ride a bike or sing “Happy Birthday,” a new constellation of connections takes shape. As she grows, every memory — a friend’s name, the feel of skis on virgin powder, a Beethoven sonata — is recorded this way. Taken together, these connections constitute her connectome, the brain’s permanent record of her personality, talents, intelligence, memories: the sum of all that constitutes her “self.” Even after the cold arrested Bagenholm’s heart and hushed her crackling neuronal net to a whisper, the connectome endured.

What makes the connectome’s relationship to our identity so difficult to understand, Seung told me, is that we associate our “self” with motion. We walk. We sing. We experience thoughts and feelings that bloom into consciousness and then fade. “Psyche” is derived from the Greek “to blow,” evoking the vital breath that defines life. “It seems like a fallacy to talk about our self as some wiring diagram that doesn’t change very quickly,” Seung said. “The connectome is just meat, and people rebel at that.”

Seung told me to imagine a river, the roiling waters of the Colorado. That, he said, is our experience from moment to moment. Over time, the water leaves its mark on the riverbed, widening bends, tracing patterns in the rock and soil. In a sense, the Grand Canyon is a memory of where the Colorado has been. And of course, that riverbed shapes the flow of the waters today. There are two selves then, river and riverbed. The river is all tumult and drama. The river demands attention. Yet it’s the riverbed that Seung wants to know.

When Seung was just shy of his 5th birthday, his father took him to their local barbershop, a screen-door joint in Austin where the vending machine served Coke in bottles. While Seung’s father was getting his hair cut, the barber stopped and pointed out an endearing scene: Little Sebastian was pretending to read the paper. “No,” his father said, “I think he’s really reading it.” The barber went over to investigate, and sure enough, the boy was happy to explain what was happening that day in The Austin American-­Statesman. Seung had taught himself to read, in part by asking his father to call out store names and street signs. At 5, he told his father — a man who escaped North Korea on his own as a teenager — that he would no longer be needing toys for Christmas.

Growing up, Seung’s primary passions were soccer, mathematics and nonfiction (with an exception made for Greek myths). As a teenager, he was inspired by Carl Sagan’s “Cosmos.” He took graduate-level physics courses as a 17-year-old Harvard sophomore and went directly into Harvard’s Ph.D. program in theoretical physics. During a 1989 summer internship at Bell Laboratories, though, Seung fell under the spell of a gregarious Israeli named Haim Sompolinsky, who introduced him to a problem in theoretical neuroscience: How can a network of neurons generate something like an “Aha!” moment, when learning leads to sudden understanding. This brought Seung to his own “Aha!” moment: At the fuzzy border between neuroscience and mathematics, he spied a new scientific terrain, thrilling and largely unexplored, giving him the same feeling physicists must have had when the atom first began to yield its secrets.

Seung became part of a cadre of physicists who deployed sophisticated mathematical techniques to develop an idea dating back as far as Plato and Aristotle, that meaning emerges from the links between things — in this case, the links between neurons. In the 19th century, William James and other psychologists articulated mental processes as associations; for example, seeing a Labrador retriever prompts thoughts of a childhood pet, which leads to musings about a friend who lived next door. As the century closed, the Spanish neuroscientist Santiago Ramón y Cajal was creating illustrations of neurons — long, slim stems and spectacular branches that connected to other neurons with long stems of their own — when people began to wonder whether they were seeing the physical pathways of thought itself.

The next turn came in more recent decades as a cross-disciplinary group of researchers, including Seung, hit on a new way of thinking that is described as connectionism. The basic idea (which borrows from computer science) is that simple units, connected in the right way, can give rise to surprising abilities (memory, recognition, reasoning). In computer chips, transistors and other basic electronic components are wired together to make powerful processors. In the brain, neurons are wired together — and rewired. Every time a girl sees her dog (wagging tail, chocolate brown fur), a certain set of neurons fire; this churn of activity is like Seung’s Colorado River. When these neurons fire together, the connections between them grow stronger, forming a memory — a part of Seung’s riverbed, the connectome that shapes thought. The notion is deeply counterintuitive: It’s natural to think of a network functioning as a river system does, a set of streams that can carry messages, but downright odd to suggest that there are parts of the riverbed that encode “Labrador retriever.”