But it gets harder. The courage of children and beasts is a function of innocence. We let our bodies go the way of our fears. A teen-aged boy, king of the world, will spend weeks in front of a mirror perfecting some difficult trick with a lighter, a muscle, a tennis ball, a coin. Why do we lose interest in physical mastery? If I feel like turning cartwheels—and I do—why don’t I learn to turn cartwheels, instead of regretting that never learned as a child? We could all be aerialists like squirrels, divers like seals; we could be purely patient, perfectly fleet, walking on our hands even, if our living or stature required it. We can’t even sit straight, or support our weary heads.

When we lose our innocence—when we start feeling the weight of the atmosphere and learn that there’s death in the pot—we take leave of our senses. Only children can hear the song of the male house mouse. Only children keep their eyes open. The only thing they have got is sense; they have highly developed “input systems,” admitting all data indiscriminately. Matt Spireng has collected thousands of arrowheads and spearheads; he says that if you really want to find arrowheads, you must walk with a child—a child will pick up everything. All my adult life I have wished to see the cemented case of a caddisfly larva. It took Sally Moore, the younger daughter of friends, to find one on the pebbled bottom of a shallow stream on whose bank we sat side by side. “What’s this?” she asked. That, I wanted to say as I recognized the prize she held, is a memento mori for people who read too much.