and his long drama, The Fall of the Heroic Life in the North (1809). In addition to a huge stream of articles and poems, he wrote a number of books, including two histories of the world (1814 and 1817), the long historical poem Roskilde-Riim (Rhyme of Roskilde) (1813), and a book-sized commentary, Roskilde Saga. Grundtvig's hymn book effected a great change in Danish church services, substituting the hymns of the national poets for the slow measures of the orthodox Lutherans. In all Grundtvig wrote or translated about 1,500 hymns, including "God's Word Is Our Great Heritage", most of which are still frequently sung today. Hans Christian Andersen (1805–1875) is remembered first and foremost for his fairy tales, written between 1835 and 1872 not only for children but for adults too. Among the most popular are "The Steadfast Tin Soldier", "The Snow Queen", "The Little Mermaid", "Thumbelina", "The Little Match Girl", and "The Ugly Duckling". Considered to be the father of the modern fairytale, Andersen wrote a total of 156 fairy stories, only 12 of which drew on folk tales. But Andersen also wrote a number of travel sketches, several novels including the well-received: "The Improvisatore" (1835), a series of poems, and his autobiography "The Fairy Tale of My Life" (1855). Søren Kierkegaard (1813–1855) was an existentialist philosopher and a theologian. Much of his philosophical work deals with the issues of how one lives, focusing on the priority of concrete human reality over abstract thinking and highlighting the importance of personal choice and commitment. His principal aesthetic works include Either/Or (Enten-Eller) (1843), Philosophical Fragments (Philosophiske Smuler) (1844), Stages on Life's Way (Stadier paa Livets Vei) (1845) and Concluding Unscientific Postscript to Philosophical Fragments (Afsluttende uvidenskabelig Efterskrift) (1846). Opposing Hegelian philosophy, they promote the existential approach which raises the individual's awareness of God but intensifies his despair at not being able to achieve eternal truth. His religious works include Works of Love (Kjerlighedens Gjerninger) (1847) and Practice in Christianity (Indøvelse i Christendom) (1850). Modern Breakthrough Modern Breakthrough was a strong Scandinavian movement covering naturalism and debating literature towards the end of the 19th century (1870–1890), replacing romanticism. The Danish theorist Georg Brandes (1842–1927) is often considered to be the "wire-puller" behind the movement. During the Modern Breakthrough, authors revolted against old traditions, especially romanticism, introducing an increasingly international outlook, a freer view of sexuality and religion, along with interest in scientific breakthroughs