Born on May 5, 1813, Søren Aabye Kierkegaard was a tall-haired theologian who brought about a sea change in Christian thought by challenging state religion and breaking with philosophical traditions that sought to prove the existence of God using logic.

He was also an enigmatic figure whose writing confounded even the wisest minds of the time (and ever since then). Raised in a household that valued intellectual life, Kierkegaard was no stranger to thoroughly and exhaustively challenging thoughts and positions. His contributions to philosophy are immense, even though he never seemed to fully agree with himself. Here are 10 things you might not have known about Søren Kierkegaard.

1. A BROKEN ENGAGEMENT AFFECTED HIS WRITING.

At 27 years old, Søren Kierkegaard was engaged to Regine Olsen, but he wrote in his journal almost immediately afterward that it was a mistake; a year later, he called it off. Some surmised that he didn’t want to share his despair and melancholic personality with anyone. It’s also possible that he decided to avoid marriage because it didn’t allow for the intensity of the philosophical project he wanted to undertake. It’s not clear exactly why he called it off, but it shook him to his soul, and he alluded to her and pled with her in his earliest writings to understand why he’d ended the relationship. The disengagement was also the launching point of a three-year period in which he published seven books.

2. HE WILLED HIS BELONGINGS TO HIS EX-FIANCÉE.

Kierkegaard saw a marriage proposal as contractually the same as a marriage, so when he died, he bequeathed his books to Olsen even though she’d married someone else years before. She did not accept the possessions.

3. HE WROTE UNDER PSEUDONYMS IN ORDER TO DISAGREE WITH HIMSELF.

A hallmark of Kierkegaard’s style of intellectual interrogation was writing under different names in order to fully examine, or sometimes contradict, the claims he made. The practice was used regularly in the late 18th and 19th centuries, with The Federalist Papers being a prime example. Kierkegaard used his own name on religious tracts that didn’t gain as much attention as his philosophical work, but the pseudonymous viewpoints still helped solidify his goal of displaying truth as subjective. All of this, according to Kierkegaard, was in service of asking the main question: how does one become a Christian?

4. HE SURVIVED COMPLETELY OFF AN INHERITANCE.

Kierkegaard’s father Michael retired at the age of 40 after great success as a wool merchant. Not only did he gift young Søren with an upbringing surrounded by thinkers and cultural figures, he left him 30,000 rixdalers, which was enough for Kierkegaard to live off of (and self-publish) for the rest of his life.

5. HE ASKED TO BE MOCKED BY A SATIRICAL DANISH PAPER.

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In 1845, Peter Ludvig Møller, a writer an editor for the satirical rag The Corsair, published a piece which criticized Kierkegaard’s Stages on Life’s Way, and Kierkegaard’s response lit a fuse on a minor feud that had a profound impact on the philosopher. In The Activity of a Traveling Esthetician and Dialetical Result of a Literary Police Action, the theologian scoffed at the paper and dared them to make fun of him. So, they did. For months they ridiculed the way he looked, talked, and acted, and the barrage of public insults humiliated Kierkegaard, but he would write later that it left him isolated in the only way that leads one to truly discover Christianity. Still, it’s not smart to attack people who buy ink by the barrel.

6. HE WAS BIG ON INDIVIDUALITY. 

G.W.F. Hegel was a dominant philosophical voice of the 19th century, espousing that reality consisted solely of what was rational. Kierkegaard’s entire philosophical program was aimed at countering Hegelian thought, opening his magnum opus Either/Or by asking, “Are passions, then, the pagans of the soul? Reason alone baptized?”

Kierkegaard also wrote against the church (specifically the Church of Denmark) as a group construct that he viewed as promoting a herd mentality that actively kept people from becoming true Christians. As if the title weren’t enough: in The Crowd is Untruth, he wrote that the formation of a crowd is to place another layer of abstraction between the individual and their personal truth. The height of all his writings extolling the virtue of individuality is probably the Knight of Faith, as seen in Fear and Trembling, who has such faith in himself and God that he can operate separately from the world.

7. HE BELIEVED FAITH IN GOD REQUIRED DOUBT.

Where Hegel sought to bring everything in the universe under the umbrella of reason, Kierkegaard approached religious faith as a paradoxical act of believing something outside the boundaries of reason. In Concluding Unscientific Postscript to Philosophical Fragments, Kierkegaard described a “qualitative leap” made by faith that recognizes there can be no sufficient amount of evidence of God’s existence that could justify the kind of total commitment that religion demands. He further concluded that faith had no substance without doubt, writing in his journal, “Doubt is conquered by faith, just as it is faith which has brought doubt into the world.”

8. HE WAS THE FATHER OF EXISTENTIALISM.

By The Royal Library, Denmark - Flickr, Wikimedia Commons

Existentialist philosophy’s core concern is the nature of man. In embracing his emotional anguish, recognizing humanity as a passionate animal, and celebrating freedom and the individual, Kierkegaard gave birth to a movement that sought authenticity in thought by reconciling abstract reason to personal experience. Subjective truth lies at the heart of existentialism, and Kierkegaard’s work went on to influence Friedrich Nietzsche, Martin Heidigger, Jean-Pau Sartre, and others.

9. HE STAYED CLOSE TO HOME.

By all accounts, Kierkegaard only left Copenhagen five times: four to go to Berlin, and once to go to Sweden. He spent his spare time attending the theater or talking to strangers on the street during walks. Even during The Corsair debacle, when he became the butt of Copenhagen’s jokes, he refused to leave town, visiting cafes and taking walks as he normally would have.

10. HE DIED YOUNG AFTER A SPINAL PROBLEM.

It’s a good thing Kierkegaard was so prolific, because he died in 1855, at the age of 42. He had developed a spinal disease (perhaps the long-gestating result of a childhood fall) and collapsed in the street. He died about a month later in Frederiks Hospital, leaving behind a dizzying array of philosophical ideas that wouldn’t make their full impact known until his writings were translated in the early and mid-20th century.