Why did Van Gogh cut off his ear?

The traditional story is that Van Gogh severed his own ear in a fit of madness after a fight with friend and fellow artist Paul Gauguin. However, a 2009 book written by German historians claimed to uncover evidence that Gauguin cut off Van Gogh's ear instead.

The 2009 book "Pact of Silence" claims that Gauguin cut off the ear during the argument, and the two men agreed to concoct the more commonly known story to protect Gauguin. The book bases this account on letters between the two men and does not conclude whether Gauguin intentionally cut the ear off or if it was an accident in self-defense.