The Sickness unto Death (Danish: Sygdommen til Døden) is a book written by Danish philosopher Søren Kierkegaard in 1849 under the pseudonym Anti-Climacus. A work of Christian existentialism, the book is about Kierkegaard's concept of despair, which he equates with the Christian concept of sin, which he terms "the sin of despair". Walter Lowrie wrote that he saw the themes in The Sickness unto Death as a repetition of those in Kierkegaard’s earlier work, Fear and Trembling, and as being even more closely related to those in The Concept of Anxiety. Kierkegaard used two pseudonyms for opposite purposes: "Johannes Climacus" suggests that he is not a Christian, whereas "Anti-Climacus" suggests he is "an extraordinary Christian". Source: Wikipedia (en)