The gist, then, is that someone is “himself” because countless mental artifacts stay firm from one day to the next, anchoring that person's character over time. It's a less crisp definition than the old idea of a soul, offering no firm threshold where selfhood breaks down. It doesn't pinpoint, for example, how many psychological chains you can lose before you stop being yourself. Neuroscience also offers only a partial answer to the question of what makes you you.

Neural networks encode our mental artifacts, which together form the foundation of behavior. A stimulus enters the brain, and electrochemical signals swoosh through your neurons, culminating in an action: Hug a friend. Sit and brood. Tilt your head up at the sun and smile. Losing some brain cells here or there is no big deal; the networks are resilient enough to keep a person's behaviors and sense of self consistent.

But not always. Mess with the biological Jell-O in just the right ways and the structure of the self reveals its fragility.

Lee's personality had been consistent for decades—until it wasn't.

From an early age, he was a person who could visualize sprawling structures in his mind. Growing up in the 1990s in Cupertino, where his dad worked at Apple, Lee had early access to the latest computers, and he and his brother grew up bingeing on videogames. As a gamer, he was legendary among his friends for being able to read a complex situation, rapidly adjust strategies, and win match after match. And it wasn't just videogames. His childhood friend Justin Powell remembers Lee strolling into a middle school chess club tournament cold. He wasn't a member of the club, but he won the tournament anyway. Lee avoided becoming insufferable by channeling his wit into snarky commentary. “Watching a movie with him was like a version of Mystery Science Theater 3000,” Powell says. “His very presence challenged you to keep up with him.”

Lee and his friends would cart their computers to each other's houses to play games together. He became curious about the machines themselves and started learning computer science, first in high school, then at a local community college and UC Santa Cruz, where an unlikely set of circumstances connected him with Matthew Prince.

Then a young entrepreneur, Prince was pursuing an idea for an antispam software tool when he encountered Arthur Keller, a UC Santa Cruz computer science professor. Keller and his students had already worked out a very similar concept. Prince and Keller agreed to share a patent, along with Keller's students. One of those students was Lee, and Prince hired him on the spot. “I had no idea this school project would turn into something much bigger,” Lee later said in a video interview with a group called Founderly.

Prince set up the company, Unspam Technologies, in Park City, Utah, about a mile from a cluster of slopes where he could indulge his passion for skiing. Lee moved into Prince's basement, at first working for free in exchange for food and housing. But Lee and the other Unspam engineers grew restless, and they started spinning up side projects, including one called Project Honey Pot, which tracked spammers as they crawled the web. That's all it did—it collected and published data on spammers, but it didn't do anything to stop them. Still, the project quickly amassed a loyal following.

In 2007, Prince left Utah to start business school at Harvard, and Lee moved to California to live with his girlfriend, Alexandra Carey. They'd known each other as undergrads, when she was a teaching assistant in his computer architecture class. Lee had goofed off in that class, once pranking the professor by scrawling childish notes on the transparencies of an overhead projector. Alexandra had been amused, but it wasn't until after college that a relationship bloomed. Living in different cities, they fell for each other while playing and chatting within a multiplayer videogame called Savage. Now, with Prince leaving Utah, it seemed a natural time for Lee to join Alexandra. They married in 2008.

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Lee and Prince kept working at Unspam from their respective cities, but as Prince was wrapping up business school, Lee called to tell him he was considering other job offers. Prince countered with a new and rather audacious pitch: He and a classmate, Michelle Zatlyn, had hit on a startup idea they thought had potential. What if they expanded Project Honey Pot to not just recognize spammers and hackers but also fight back against them? The plan was to build out massive networks of servers around the world, convince website owners to route their traffic through those servers, and gather enough data to detect malicious requests amid the good ones. That might give them the tools they needed to stop even the world's biggest denial of service attack. But Prince needed a technical cofounder, and his about-to-defect employee was his top choice.