Danish philosopher, theologian, and literary stylist who wrote furiously for a decade in Copenhagen in the 1840s and early 1850s, largely ignored in his lifetime, and who is now recognized as the single most important source of existentialist thought. His work was a one-man campaign against the Hegelian system, against the bourgeois Christianity of the Danish state church, and against the pretension that human existence could be captured by any objective philosophical system whatsoever.
Kierkegaard's signature move is the use of pseudonymous authorship — Johannes de Silentio, Johannes Climacus, Anti-Climacus, Victor Eremita — each a distinct voice staging a different way of existing, so that the reader is drawn into the question of how she means to live rather than being handed a doctrine. Fear and Trembling (1843) took up the story of Abraham and Isaac to probe the "teleological suspension of the ethical" — the possibility that faith demands something no universal moral rule can authorize. The Sickness Unto Death (1849) diagnosed despair as the condition of the self not yet willing to be itself before God. Concluding Unscientific Postscript (1846) argued, against G. W. F. Hegel, that existential truth cannot be held in the mode of detached speculation: "Truth is subjectivity."
His three "stages on life's way" — aesthetic (pursuit of the interesting), ethical (commitment to the universal), religious (the individual before the absolute) — are neither chronological nor simply ranked; they are modes of existence one chooses, or refuses to choose. His influence on 20th-century theology (Paul Tillich, Barth, Bultmann) and on atheist existentialism (Sartre, Camus) is hard to overstate.