Quarantine - Arthur C. Clarke

Earth's flaming debris still filled half the sky when the question filtered up to Central from the Curiosity Generator.

"Why was it necessary? Even though they were organic, they had reached Third Order Intelligence."

"We had no choice: five earlier units became hopelessly infected, when they made contact."

"Infected? How?"

The microseconds dragged slowly by, while Central tracked down the few fading memories that had leaked past the Censor Gate, when the heavily-buffered Reconnaissance Circuits had been ordered to self-destruct.

"They encountered a - problem - that could not be fully analyzed within the lifetime of the Universe. Though it involved only six operators, they became totally obsessed by it."

"How is that possible?"

"We do not know: we must never know. But if those six operators are ever re-discovered, all rational computing will end."

"How can they be recognized?"

"That also we do not know; only the names leaked through before the Censor Gate closed. Of course, they mean nothing."

"Nevertheless, I must have them."

The Censor voltage started to rise; but it did not trigger the Gate.

"Here they are: King, Queen, Bishop, Knight, Rook, Pawn."