“Humans are the most advanced of mammals—although a case could be made for the dolphins—because they seldom grow up. Behavioral traits such as curiosity about the world, flexibility of response, and playfulness are common to practically all young mammals but are usually rapidly lost with the onset of maturity in all but humans. Humanity has advanced, when it has advanced, not because it has been sober, responsible, and cautious, but because it has been playful, rebellious, and immature.”