In the fall of 1893, a young German man fell from his horse.

Hans Berger, nineteen years old. He was in the middle of military training. His body slammed into the ground, and the wheels of the artillery unit behind him grazed his head. He came within centimeters of death.

That evening, hundreds of kilometers away at home, his sister was seized by sudden anxiety. An inexplicable terror washed over her. She convinced their father to send a telegram. "I think something has happened to Hans."

By the time the telegram arrived, Hans Berger was already safe. But a single question had carved itself into his mind. How did my fear reach my sister? Can thoughts be transmitted?

Thirty-one years later, on July 6, 1924, Berger began searching for an answer in an operating room at Jena University Hospital. He placed electrodes on the exposed brain of a patient whose skull had been opened, and the galvanometer needle trembled. The brain was sending electrical signals. Thought had a physical reality.

Berger did not publish his discovery for five years. He feared his colleagues would ridicule him. When his paper finally appeared, the academic response was cold. Yet today, electroencephalography (EEG) is used daily in hospitals around the world. Berger's obsessive question gave humanity its first window into the brain.

Exactly one hundred years later, on January 29, 2024, a different kind of window opened in a hospital in Phoenix, Arizona.

Noland Arbaugh, age 29. Eight years earlier, he had broken his neck diving into a lake. From that day on, he felt nothing below his shoulders. He could not move a single finger.

In the operating room, a robot called R1 drilled a hole in his skull. About the size of a 500-won coin. Through that hole, a coin-sized chip was inserted. Attached beneath the chip were 64 threads thinner than a human hair, each carrying 16 micro-electrodes. A total of 1,024 electrodes were implanted into his motor cortex.

The day after surgery, Arbaugh stared at a laptop screen in front of him. The cursor moved. He had not used his hands. He moved it with thought alone.

The device, made by Neuralink, is called Telepathy.

The question Hans Berger had devoted his life to, whether thoughts can be transmitted, had just received its first practical answer.

This book is the story of brain-computer interfaces (BCI).

What is a BCI? It is a technology that directly connects the human brain to a computer. It reads the electrical signals the brain sends and relays them to a machine; sometimes it sends signals from a machine back into the brain, creating a two-way channel. It is a technology that lets a person steer a wheelchair with thought alone, lets someone who cannot speak type sentences with thought alone, and lets a blind person see light by directly stimulating the visual cortex.

It sounds like something out of a science fiction novel. But as of 2025, this is a reality undergoing clinical trials. Patients in the United States, the United Kingdom, Canada, and the United Arab Emirates are having chips implanted in their brains. The first patient, Noland Arbaugh, plays chess with his thoughts, plays Civilization 6, and uses the device for eight consecutive hours. The second patient, Alex, creates 3D designs in CAD software. Brad, a patient with ALS, has edited YouTube videos using only brain signals, and an artificial intelligence restored his voice so he could talk with his family.

In September 2024, the U.S. Food and Drug Administration (FDA) granted Neuralink's speech restoration technology a Breakthrough Device designation. Its vision restoration device, Blindsight, received the same designation. The conversation between brain and machine had officially begun.

But this book is not a technical manual.

I have come to hold one conviction: great innovation is born at the intersection of technology and the humanities. And at that intersection, there is always a human being.

Why is Neuralink's founder, Elon Musk, obsessed with the brain? He fears that artificial intelligence will surpass humanity. The amount of information the human brain can transmit per second is a mere few dozen bits. That is roughly the speed of typing a text message on a smartphone. Machines, by contrast, process billions of bits per second.

Musk's logic goes like this: if we don't close this bandwidth gap, humans become pets of artificial intelligence. So the brain and computer must be connected directly.

Why did the first patient, Noland Arbaugh, want a chip implanted in his skull? He is a young man who could not move a single finger for eight years. For him, risk was not an abstract concept. "What do I have to lose?" he said in a pre-surgery interview.

Why did Chilean Senator Guido Girardi push to enshrine neurorights in the constitution, a world first? Because he sensed that brain data would become the new oil. A future where your thoughts, your emotions, your memories become corporate commodities. He tried to prevent that future.

This book tells the stories of these people. The people who built the technology, the people who accepted it into their own bodies, the people who stand guard against its shadows. Through their motivations, fears, obsessions, and hopes, I seek the meaning of BCI.

At the same time, this book is a history.

Every innovation stands on the attempts that came before it. From Hans Berger's discovery of brain waves in 1924, to Jacques Vidal's proposal of the BCI concept in the 1970s, to BrainGate's first human clinical trial in 2004, to Neuralink's Telepathy in 2024. A journey of one hundred years.

Neuralink is not alone. Australia's Synchron inserts electrodes into the brain through blood vessels, without opening the skull. Blackrock Neurotech, which originated at the University of Utah, holds the record for a long-term implant that has operated for more than nine years. Precision Neuroscience is developing an ultra-thin film that adheres to the brain's surface. Meta is researching a non-invasive interface through an electromyography wristband worn on the arm.

China is also in pursuit. A state-led initiative called the China Brain Project is underway. As of 2024, 664 companies are competing in the BCI field, with 2,160 patent families on record.

It is a Warring States era. This book draws the map of that competition.

And this book is a collection of questions.

At the door that BCI has thrown open, there are more questions than answers.

What is mental privacy? Who owns your brain data? According to a 2024 survey by the NeuroRights Foundation, 29 out of 30 companies manufacturing consumer-grade brainwave devices held unlimited access rights to their users' brain data. Ninety-seven percent of those companies included clauses allowing them to transfer brain data to third parties. An era in which your thoughts become someone else's commodity is approaching.

What about the possibility of brainjacking? Suppose someone accessed your motor cortex and moved your hand. Suppose they hacked your visual cortex and made you see things that were not there. For now, this remains science fiction. But the moment the brain connects to a machine, it becomes a theoretical possibility.

Where is the line between therapy and augmentation? Restoring motor function to a paralyzed patient is therapy. Then what about doubling a healthy person's memory capacity? Boosting concentration? Increasing language-learning speed fivefold? If only the wealthy can access such augmentation technologies, wouldn't a new species of inequality be born?

In 2021, Chile became the first country in the world to amend its constitution to protect neurorights. In 2024, California passed a bill classifying brain data as sensitive personal information. In 2025, the UN Human Rights Council Advisory Committee urged all nations to regulate neurotechnology. Efforts to close the gap between law and technology had begun.

This book deals with all of those questions honestly. I don't have the answers. I won't force any, either. I simply want to think them through together with you, the reader.

Last of all, this book is preparation for what lies ahead.

Neuralink is targeting mass production by 2026. Elon Musk says that within a few years hundreds, within five years tens of thousands, and within ten years millions of people will have chips implanted in their brains. It may be an exaggeration. His predictions have often missed the mark. But the direction is clear.

An era in which the boundary between brain and machine grows faint is coming.

Are we ready? Can we enjoy the benefits of a medical revolution while still protecting our privacy? Can we draw an ethical line between therapy and augmentation? Can we prevent technological divides from creating new classes? Can we tell exaggeration from fact when we encounter BCI news?

The final chapter of this book contains ten questions for evaluating BCI news. I hope we can think together about what civic literacy means in the age of brain data.

Hans Berger set out to find telepathy and ended up inventing the electroencephalogram. He believed thoughts could be transmitted, yet what he actually discovered was that thoughts have an electrical reality.

A hundred years later, we hold in our hands the technology to read, interpret, and transmit those electrical signals. Berger's dream of telepathy has begun to come true.

But what is true telepathy? Is it transmitting the language inside our heads through a machine? Or is it the effort to genuinely understand each other's thoughts?

Technology is only a tool. How we use that tool is our choice.

I hope this book serves as a small compass for making that choice.

January 2026, Kyungjin Kim