A remarkable number of great philosophers living before the 20th century were unmarried and childless, with erotic love seeming to have had little impact on their ideas. But the philosophy of Søren Kierkegaard, the 19th-century Danish existentialist, is famously bound up with his almost-marriage to Regine Olsen. His voluminous Either/Or, begun soon after he broke off his engagement to her, is full of covert but revelatory references to their relationship, as are several of his other books. 

Kierkegaard probably met Olsen in 1837, when he was twenty-four and she fifteen. He proposed three years later, but on the very day she accepted his offer he wrote in his journal that he knew he had made a mistake. Over the following year, the passionate intensity of the letters