Moolah took us somewhere new.

It was a co-working space in Nakano, up three flights of stairs and behind a door marked only with a small brass number. Scattered inside are long wooden tables, cheap monitors, a kettle, and a cat that had decided it lived there. About fifteen people were working on laptops with the quiet focus of a library. Nobody looked up when we came in. Nobody looked up when Moolah padded to the kettle and helped herself to hot water with the easy air of a regular.

"I wanted you to see this," she said, dunking a teabag, "before we talked about Ethereum properly."

"Why?" I asked.

"Because Ethereum is one of these." She gestured at the room.

Dev and I looked around. The cat yawned.

"I've been telling you Ethereum is a shared notebook," Moolah said, settling onto a stool at our table. "Which is true, but only partially. A notebook is just records. Ethereum is something stranger - it's a shared computer."

"A computer."

"A single computer that everyone in the world can run programs on, at the same time, with everyone agreeing on the results. Look around. This room is full of individual people working on individual laptops. Now imagine that instead of separate laptops, all fifteen of them shared one machine - with their own keyboards and mouses - and any one of them could type a command and everyone would see the result. That's closer to what Ethereum is. It's a computer shared by the whole world."

"That sounds…" I searched for the word. "Inefficient."

"Catastrophically inefficient," Moolah agreed cheerfully. "Ethereum is one of the slowest, most expensive computers ever built. A phone from 2008 is faster. A microwave has more computing power. And yet it is also the most trustworthy computer ever built, because every instruction it runs is verified by thousands of independent machines, and the results are written into a ledger that no single party can rewrite."

"So what's the tradeoff?"

"Speed and cost, for trust. A regular computer is fast and cheap, but it belongs to someone - a company, a person, a government. You have to trust the owner. Ethereum doesn't belong to anyone. Nobody can unplug it. Nobody can reboot it. Nobody can quietly change what it does. That unownability is the product and computing feels almost like a side effect."

Dev, who had been watching me take notes, jumped in. "This is why people call it a world computer."

"A dramatic name," Moolah said. "But reasonable. Think of it like a public library. Anyone can walk in, use the computer lab, leave a program running on one of the machines, and know it will still be running in a year, regardless of who the librarian is, because just like anyone can use this computer, anyone can be a librarian."

"And smart contracts -"

"Are programs left running in that library. Anyone can start one. Anyone can use one. Once it's running, no single party can shut it down, change its rules, or charge admission to interact with it. It sits there forever, doing what its code says it does, for as long as Ethereum itself exists."

I thought about the corner of the co-working space where the cat was now kneading a jacket. "So if I wrote a program today and put it on Ethereum, it would still be there in a hundred years."

"If Ethereum is still there in a hundred years, yes. Which is the other wager. You're not just trusting the computer, but also the promise that the network of people running it will keep running it. Which they will, as long as enough of them want to."

"And they want to because -"

"Because running the computer pays. Remember the gas fees from last week? Those fees go to the people running the machines. As long as the network is useful enough that people pay to use it, the operators keep showing up. It's an economy that sustains itself by being useful."

Dev had been quiet. Now he said, "This is the part I couldn't explain to you before."

"I know."

"Because it sounds ridiculous out loud. A world computer. Most people hear that and assume you've joined a cult."

"It is a little ridiculous," Moolah said. "That's why it's been so hard to explain for so long. It doesn't map to anything people already know. It's not a faster version of the internet. It's not a cheaper version of banking. It's a new kind of machine, and new machines take a long time to find their vocabulary. The printing press was called a 'mechanical scribe' for its first fifty years. Electricity was called 'captured lightning.' Ethereum is, right now, in its mechanical-scribe decade."

I wrote A WORLD COMPUTER at the top of a fresh page. It still sounded ridiculous. But for the first time, it also sounded accurate.

Under it, I wrote: slow and expensive on purpose. The trust is the product.

Moolah drank her tea. The cat, having finished kneading Dev's jacket, climbed onto the table and lay across my notebook like a furred paperweight.

"Well," Dev said, watching the cat settle, "I guess we're done for the day."