Not long after the founding of Joseon, Neo-Confucian scholarship — which had taken root in the late Goryeo period — grew into the central discipline of the royal court, thanks to great scholars like Toegye Yi Hwang. King Seonjo trusted Yi Hwang deeply, and when the aging scholar prepared to retire, the king asked him, "If you step down, whom shall I turn to for counsel on matters of state?" Without hesitation, Yi Hwang recommended a minister named Yi Jun-gyeong.

At the time, the court was largely split into two groups: those who rose to office through recommendation — often descendants of great families or royal relations — and those who earned their positions through the state examinations. Yi Jun-gyeong himself had passed the examinations, but he promoted talented men regardless of their background. This didn't sit well with scholars who saw themselves as a special class, and many came to resent him — including Yulgok Yi I, who would later become a towering scholar in his own right.

Before his death, Yi Jun-gyeong submitted a final memorial to the king. In it, he wrote: "I observe that men today form cliques, exclude one another, and delight in grand and lofty talk — this custom must be broken." Yi I took this as an attempt to turn the king against the scholarly class altogether, and thought poorly of him for it.

But Yi Jun-gyeong's judgment proved sharp. Though not yet visible on the surface, the seeds of factionalism were already taking root in the court.

Some time before this, a court official happened to visit the home of a chief minister and noticed the bedding of a young scholar named Kim Hyo-won, who was staying there as a guest. Kim Hyo-won was known as talented and promising, and the official privately looked down on him for lodging as a client in a powerful minister's house.

Later, when Kim Hyo-won passed the state examinations and rose in reputation, guiding younger scholars and earning their respect, the same official kept calling him petty behind his back, citing that old incident. The younger scholars pushed back — and it turned out Kim Hyo-won had stayed there not as a client, but because of a family relation. The tide of public opinion turned against the official instead.

This is how the older and younger generations came to clash and split apart. Because the two men's homes stood on the east and west sides of the capital, their respective factions came to be known as the Easterners and the Westerners.

The man who played the largest role in this growing chaos was Yulgok Yi I. Born into a modest scholarly family, he had briefly turned to Buddhism before returning to Confucianism, eventually mastering Neo-Confucian philosophy at its highest level. The king trusted him deeply, and though the two frequently argued over matters of scholarship, the king never let him stray far from his side.

At first, Yi I tried to mediate between the two factions. But over time, he drifted naturally toward the Westerners, as the younger scholars grew increasingly extreme and the Easterners — who now held most of the court's key positions — began impeaching Westerner officials one after another. Yi I worked hard to end this pointless conflict, but never succeeded, and he passed away without seeing it resolved.

After Yi I's death, in the gichuk year of King Seonjo's reign, a major incident erupted.

There lived in Jeonju a man named Jeong Yeo-rip. He had once studied under Yi I and was known for his talent, but later turned his back on his teacher and joined the Easterner faction, eventually abandoning his post and retreating to his hometown. There, he gathered disciples and taught a dangerous doctrine: "Any king can be served faithfully — loyalty is not owed to bloodline alone."

That October, Jeong Yeo-rip's plot to rebel was discovered. He fled into the mountains with his son and two followers, and when government troops surrounded them, he took his own life.

But the real trouble was only beginning. Anyone who had even the faintest connection to Jeong Yeo-rip began to be arrested one by one. Even ministers of the Easterner faction, who held prominent positions at court, were forced from office simply for having once been close to him.

The task of investigating the affair — serving as chief inquisitor — fell to a Westerner named Jeong Cheol. Jeong Cheol was known as a stern, uncompromising man. Yet remarkably, records show he made repeated efforts to save Easterner officials — his political opponents — during this very investigation.

When the State Councilor Jeong Eon-sin faced execution for his ties to Jeong Yeo-rip, Jeong Cheol told the king, "In two hundred years, our court has never once executed a senior minister." The king withdrew the death sentence and reduced it to exile.

The same happened with a man named Choi Yeong-gyeong, who fell gravely ill in prison. Jeong Cheol repeatedly sent physicians to tend to him. Even so, Choi Yeong-gyeong ultimately died in captivity — and the question of who bore responsibility for his death would become a seed of controversy that lasted for generations to come.