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This is stated in the Book of Five Rings, written by samurai Miyamoto Musashi circa 1645. Musashi was considered a "sword saint" and an almost supernatural samurai, having never lost a battle. He invented several sword fighting techniques, the most famous of which was the Niten Ichi-ryΕ«. He wrote the book as a sort of reckoning on why he had been so lucky to survive and analyzing the current sword styles.