In 1889, Nietzsche suffered the dramatic breakdown that would debilitate him until his death 11 years later: Upon catching sight of a man flogging a horse in a public square in Turin, the story goes, he threw his arms around the animal’s neck, burst into tears, and crumpled to the ground. He had already displayed signs of volatility before this collapse. According to Kaag, Nietzsche “began to sign his letters ‘Dionysus’ ” in 1888, and he had a troubled relationship with food throughout his life, ricocheting from one extreme diet to the next. As he grappled with the specter of decadence, his austere and itinerant life represented a rejection of the indulgent spirit dulling the haute bourgeoisie of fin de siècle Europe.

At 19, Kaag shared Nietzsche’s distaste for the “scripted” routines and glib gratifications that make modern life so deplorably easy. When he arrived in Basel on a research trip, he recoiled at his surroundings: The train station was “a model of Swiss precision,” and the streets were “too straight, too quiet, too mundane.” He quickly abandoned his plans to ruminate on Nietzsche from the confines of Basel’s library and instead decided to follow his idol’s grueling alpine route to “Splügen, then to Grindelwald at the foot of the Eiger, then to the San Bernardino Pass, to Sils-Maria, and finally to the towns of Northern Italy.” He wanted, he writes, “to feel something, to break through the anesthesia, to prove to myself that I wasn’t just asleep.”

How did “Nietzsche cultivate the existential defiance or courage that led [him] up the mountain?” Kaag asks.

It probably started something like this—in a very simple refusal to act on behalf of one’s obvious self-interest. There remains a life-affirming glee in such a refusal—a quiet temptation that even the most well-adjusted person feels at various points.

As Kaag advanced along Nietzsche’s trek, his refusal started to take the form of fasting so intense, it often left him dizzy. When he finally relented, he stumbled into a luxury hotel and ordered a sickeningly opulent multicourse meal.

His reversal was radical—and, in some ways, utterly Nietzschean: “The spell that fights on our behalf, the eye of Venus that charms and blinds even our opponents, is the magic of the extreme, the seduction that everything extreme exercises,” Nietzsche wrote in The Will to Power, the posthumously published book that many consider his magnum opus. This core provocation, the heart of his teachings and his notoriously hyperbolic style, is by turns enthralling and juvenile.

Mercifully, 37-year-old Kaag is aware of the raw, adolescent quality of some of Nietzsche’s most radical insights, and he acknowledges that during his own infatuation with his hero he was “thoroughly intolerable.” Still, he isn’t entirely immune to the seductions of Dionysian extremity. When he revisits the Swiss town of Sils-Maria 18 years after his initial trip, he both laments and celebrates his maturation. He smiles for a family photo and entertains fleeting worries that he has become “a grinning domesticated animal,” one of the pampered. How to resist? When he heads back to his family after a day of lonely hiking, he reflects, “Perhaps a pilgrim triumphs not in hardship, but in the rare moment when they learn to accept something soft at home.” This sounds like an un-Nietzschean lesson, and Kaag’s clichés can sometimes leave us wondering whether he embraces Wittgenstein’s vision of philosophy after all.