Tokyo does not generally unleash enraged diplomatic broadsides on friendly nations, but the debunking of Ryuta Kawashima and Nintendo could prove a thorny moment in Franco-Japanese relations.

Dr Kawashima, neuroscientist and all-round grey matter guru, is on an extraordinarily high pedestal in Japan. Commuter trains hum to the sound of middle-aged middle managers attempting to cheat the march of time on their DS consoles. The things are passed around in schools and old folks' homes as a sort of digital medicine. Toyota has co-opted the good doctor to help to design a car that would prevent elderly drivers from getting into scrapes.

Dr Kawashima's theories have seeped indelibly into Japan's nervous worldview. As a nation we are getting older, runs the credo, but if we follow Dr Kawashima at least we won't lose our marbles. It will take a good deal more than a Frenchman to rattle that faith.

In any case, gurus are definitely not to be toyed with in Japan. This is a nation where religion is weak but belief is astonishingly strong. Millions believe wholeheartedly in a guru-propounded theory that personality, fortune and future are determined by one's blood group.

No amount of foreign debunking (and there has been plenty) has rattled this. The morning news show on Japan's popular Fuji TV still concludes with a daily forecast along the lines of: “Tuesday - a good day for As, a bad day for Os. AB negatives best stay indoors.”

Similarly, Kazuho Hosoki, a television fortune teller, commands a remarkable following despite some spectacularly rotten prognostications: on a live new year broadcast in 2005 she famously told the internet entrepreneur Takafumi Horie that he was on course for a fantastic year. Three weeks later he had been arrested, his company had collapsed and thousands of investors were ruined.

With a charismatic guru and a plausible yarn at the helm, there is no limit to the directions in which Japanese may be steered. Last year, after a diet guru proclaimed that two bananas eaten every morning with a glass of tepid water would help one to lose weight, no matter what else one ate during the day, the supermarkets were stripped of bananas.