A pottery teacher in Kyoto has a habit that confuses her new students. When she holds up a finished bowl, she doesn't point to the clay. She points to the air inside.

"This is what the bowl is for," she says. She's said it hundreds of times, and she means it every time.

One student — a graduate student in engineering, precise and a little impatient — pushes back. "But the clay is the bowl. Without the clay there's nothing to hold the space."

The teacher sets the bowl down. "Right," she says.

She picks up a lump of raw clay, heavy and solid, and sets it on the wheel. She works it in silence for a while. The student watches her hands, the way the clay rises and opens under her palms. A wall forms. Then a rim. Then, suddenly, there is a bowl where there was just clay — except that what makes it a bowl is not the clay, which was always there, but the hollow that wasn't.

"The clay limits the space," the teacher says at last. "The space makes it useful."

The student takes the bowl home and puts it on his desk. He looks at it while he's thinking. He isn't entirely sure what changed, but something did.

Kū is like that. Not an absence you can point to. More like the thing you finally notice when you stop looking at the form.