Yi I (Yulgok)

Yi I, known by his pen name Yulgok, was the second great Korean neo-Confucian philosopher and the chief intellectual rival of Yi Hwang. In the Four-Seven Debate, he challenged Toegye's position, arguing for a more unified view in which principle and material force cannot be separated—material force does the activating, while principle provides the norm.

Yulgok was also a prominent statesman and reformer who attempted to put Confucian governance into practice, advocating for tax reform and military preparedness against Japanese invasion. He is celebrated both as a philosopher of systematic rigour and as a practical thinker concerned with applying moral principles to political life. His image appears on the Korean 5000-won note, and he remains among the most revered intellectual figures in Korean history.