THERE is no place on Earth where a person can say with absolute certainty that they are not being stalked by ninjas. Common sense suggests this is unlikely, but pure logic dictates that you cannot prove a negative, and the art of the ninja is to go unperceived. I have been around the world to look for them, to shadow them in reverse, and whenever I find a possible candidate, he or she tends to deny it. “No no no,” said Mats Hjelm, a web-designer from Stockholm, during a short break from his ninjutsu class in Tokyo. “I don’t like to call it by that name, although I know that some other people do. And I definitely don’t call myself a ninja.” This was, of course, exactly what a ninja would say.