The Return of Sherlock Holmes

"Well, I thought the same," said Lestrade. "But this Morse Hudson is the main supplier of these busts in that part of London. The three busts had been in his shop for years. So, although you are right to say that there are many hundreds of statues in London, it is quite likely that these three were the only ones in the area. Therefore, a fanatic who lives nearby would begin by attacking them. What do you think, Dr. Watson?"

"There are no limits to what someone with a mental illness might do" I answered. "There is an illness which modern French psychologists call the "idee fixe". This may be something totally unimportant, and the person who has it might be completely normal in every other way. For example, a man who had read a lot about Napoleon, or who possibly knew someone in his family who was injured in the Napoleonic wars - a man like this might get an idee fixe. And because he has this problem he might be capable of any kind of strange or dangerous behaviour."

"That cannot be right, my dear Watson," said Holmes, shaking his head, "for no amount of idee fixe would allow your interesting madman to find these busts in the first place."

"Well, how do you explain it?"