Søren Kierkegaard

The father of existentialism was a Christian, and for a century secular existentialism lived off an inheritance while disowning the man who left it. Kierkegaard grew up in Copenhagen under the severe piety of a wealthy father; he fell in love with Regine Olsen, became engaged, broke it off himself, and wrote about the wound for the rest of his short life. He published most of his major books under invented names – Johannes de Silentio, Climacus, Anti-Climacus – each defending its own way of living, as if the author had dealt himself out to a whole cast.

A person, for Kierkegaard, passes through stages of existence: the aesthetic (a life lived for the moment and its pleasures), the ethical (a life of duty), and the religious, which only a leap can reach. In Fear and Trembling he stares at Abraham raising the knife over Isaac and finds a teleological suspension of the ethical – the moment when the believer hears a call higher than any general rule. Against the crowd, which averages everyone down and lets no one answer for himself, he sets the single individual: "the crowd is untruth."

At the end he turned on the established Danish church for swapping Christianity for respectability, and he did it in the name of Christianity itself. He died exhausted at forty-two. This site already has Nietzsche, Heidegger, Sartre, and Camus; here is the believer all of them were arguing with, whether they named him or not. Remove Kierkegaard and they lose the interlocutor who asked their questions first.