Take the concept “Money,” for example. You can’t learn it simply by viewing a piece of colored paper, a gold nugget, a seashell, and a pile of barley or salt, each of which has been deemed currency by some society in history. Likewise, instances of an emotion category such as “Fear” don’t have enough statistical regularity—as demonstrated in chapter 1—to allow a human brain to build a concept based on perceptual similarities. To build a purely mental concept, you need another secret ingredient: words.
