Immaterialism

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George Berkeley proposed that material things exist only because they are perceived to exist. In 1993, L.C. Rodó offered a suitably Berkeleyan puzzle in El Acertijo. Any piece, including a king, that is not regarded (attacked or guarded) by another piece disappears from the board. In the diagram above, every man is either attacked or guarded. But a single move (it may be by either White or Black) will set in motion a chain reaction in which all the pieces vanish from the board. What is the move?

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