never seen or even heard of a rhinoceros, Polo reluctantly concluded that he had met his first unicorn. Admittedly, the ugly brute looked nothing like the animals “as we describe them when … they let themselves be captured by virgins.”4 But what else could it be? Here is Wineburg’s troubling conclusion: each of us naturally follows the exact same cognitive process that Marco Polo was unconsciously employing when he mislabeled those Sumatran rhinos. And because when we delve into primary sources we