Perception may seem effortless, but in fact it requires considerable energy. Each precious calorie you burn on perception is a calorie you must find and take from its owner—perhaps a potato or an irate wildebeest. Calories can be difficult and dangerous to procure, so evolution has shaped our senses to be misers. One consequence, we discover in chapter nine, is that vision cuts corners: you see sharp detail only within a small circular window, whose radius is the width of your thumb held at arm’s length. If you close one eye and hold out your thumb, you can see just how tiny it is. We think we
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