Pure Interaction Terms: The Witches

In his brilliant book The Witches, Roald Dahl gives a probabilistic description of witch features:

How to Recognise a Witch

The next evening, after my grandmother had given me my bath, she took me once again into the living-room for another story. “Tonight,” the old woman said, “I am going to tell you how to recognise a witch when you see one.” “Can you always be sure?” I asked. “No,” she said, “you can’t. And that’s the trouble. But you can make a pretty good guess.” She was dropping cigar ash all over her lap, and I hoped she wasn’t going to catch on fire before she’d told me how to recognise a witch. “In the first place,” she said, “a REAL WITCH is certain always to be wearing gloves when you meet her.”

“Surely not always ,” I said. “What about in the summer when it’s hot?” “Even in the summer,” my grandmother said. “She has to. Do you want to know why?” “Why?” I said. “Because she doesn’t have fingernails. Instead of fingernails, she has thin curvy claws, like a cat, and she wears the gloves to hide them. Mind you, lots of very respectable women wear gloves, especially in winter, so this doesn’t help you very much.” “Mamma used to wear gloves,” I said. “Not in the house,” my grandmother said. “Witches wear gloves even in the house. They only take them off when they go to bed.”

And he goes on with wigs, scratching scalps, nose holes, their eyes, blue spit and the missing toes 🙂 Like an experienced data analyst he points out the power of pooling evidence:

“None of these things is any good on its own,” my grandmother said. “It’s only when you put them all together that they begin to make a little sense.

A splendid narration of this chapter can be found here: