Danish philosophy was dominated in the first half of the 19th century by the influence of Hegel and Hegelianism. Johan Ludvig Heiberg (1791–1860), Frederik Christian Sibbern (1785–1872), and especially Hans Lassen Martensen (1803–1884), all of whom contributed to the popularity of Hegel's idealism in various academic disciplines, though Hegel's influence significantly declined by 1850. The primary critic of Hegelianism, and the most important philosopher in Denmark at the time, was Søren Kierkegaard (1813–1855), an existentialist philosopher and a theologian. Much of Kierkegaard's 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). Another important figure in Danish philosophy was N. F. S. Grundtvig (1784–1872), whose ideas became an important part of the development of Danish national identity.