Yi I (26 December, 1536-1584) was one of the two most prominent Korean Confucian scholars of the Joseon Dynasty, the other being his older contemporary, Yi Hwang (Toegye). Yi I's mother was Sin Saimdang, herself an accomplished artist and calligraphist. Yi I is often referred to by his pen name Yulgok (율곡; 栗谷 "Chestnut valley"). His courtesy name was Sukheon (숙헌; 叔獻)
Yi I was born on 26 December 1536 in Bukpyeong, in Gangwon Province. He was a child prodigy who knew Chinese script at the age of three and composed poems in Classical Chinese before he had reached his seventh birthday. By the age of seven, he had finished his lessons in the Confucian Classics, and he passed the Civil Service literary examination at the age of 13.
At the age of 29, Yi I passed a higher Civil Service examination—with full marks—and he started government service. His winning thesis, titled Cheondochaek, was widely regarded as a literary masterpiece, displaying his knowledge of history and the Confucian philosophy of politics, and also reflecting his profound knowledge of Taoism.
At 34, Yi authored "Dongho Mundap", an eleven article treatise devoted to clarifying his conviction that righteous government could be achieved even within his own lifetime, showing his aspirations and also measures to achieve it.
Yi temporarily renounced the world by secluding himself in the Diamond Mountain following his mother's death when he was 36. It is not known why he did this, although it is thought that either: he sought three years of lamentation until the Buddhist phrase, "life is transient", eased his sorrow; he may have understood that the Confucian teaching, "preserve your mind and nurture your nature", was synonymous with the Buddhist teaching, "open your mind and see your nature"; or he may have regarded it as a pleasure simply to retire to the countryside to rest.
Following his return to society, he authored "The Essentials of Confucianism" in 1576, which was considered to be a most valuable book, showing examples for a good Confucian life.
Yi died in 1584, and the valuable Yulgok Jeonjip ("The Complete Works of Yulgok") was compiled after his death on the basis of the writings he bequeathed.
He is also well-known for his foresight about the national security of Joseon Dynasty. He proposed to draft and raise 100,000 men against possible Japanese attack. Rejected by central government, his worry was soon found to be true after his death when Hideyoshi Toyotomi's Japanese force invaded Korea in 1592. The "Yulgok Project", a recent modernization project for South Korean armed force, was also named after him.