"So I have to husband what seconds I have. When I fight, I rarely switch on the Possible Sword. For the most part, I fight with it as a dumb, purely factual weapon: a diamond-hard blade with edges finer than honed metal. And I wield it precisely. Every strike I make is exact, and lands where I wish it to land. It’s what I trained for so many years to do."

Bellis could hear no pride in his voice.

"But when the situation’s severe, when odds are very bad, when a display’s needed, or I’m in danger… then I switch on the motor for a few seconds. And in that situation, precision is the one thing I cannot afford."

He was silent as a gust of warm wind shook the trees, making them sound as if they shivered at his words.

"A headsman knows where his blade must land. With every nuance of skill, he aims for the neck. He narrows the possibilities. If he were to use a Possible Sword, the vast bulk of the nighs would exist within an inch of the factual strike. The rub is this: the better the headsman, the more precise his strike, the more constrained potentiality, the more wasted the Possible Sword. But, obviously, put a weapon like this in the hands of an amateur, it’s as lethal to him or her as to any quarry-the possibilities that’ll manifest include self-harm, unbalancing, dropping the weapon, and so on. A middle way is needed.

"When I attack with a dumb weapon, I’m an executioner. My blade lands in the space I decide, and not to either side. That’s how I learned to fight; it would be a stupid waste of power to use the Possible Sword so. So when I finally found it, after a very long time of searching, I had to learn swordsmanship again. A very different art: skill without precision.

"Fighting with a Possible Sword, you must never constrain possibilities. I must be an opportunist, not a planner-fighting from the heart, not the mind. Moving suddenly, surprising myself as well as the opponent. Sudden, labile, and formless. So that each strike could be a thousand others, and each of those nigh-swords is strong. That’s how to fight with a Possible Sword.

"So I am two swordsmen."

When his lovely voice ebbed away, Bellis was aware again of the surrounds of the park, the warm darkness and the noise of roosting birds.

"What’s known about possibility mining," he said, "I know. That’s how I knew of the sword."

Uther Doul was stirring things in Bellis’ mind. In New Crobuzon, during her time when the scientist Isaac was her lover, Bellis had observed his obsessions, and had learned certain things.