Intellectual setting
Yulgok served at the Joseon royal court in Hanseong under King Seonjo during a period of growing factional division among the sarim literati and mounting external threats that would culminate, after his death, in the Japanese invasions of the 1590s; his repeated warnings about military unpreparedness went largely unheeded.
Key ideas
"Only vital force issues; principle mounts it"
Against T'oegye, Yulgok argued in his letters to Song Hon that only qi (vital force) actually issues or moves, with li (principle) serving as the norm or pattern that qi necessarily follows, so the Four Beginnings are not a separate moral source but a proper subset of the Seven Emotions when they conform to principle.
Sage learning as practical statecraft
Yulgok held that self-cultivation and government were inseparable, producing works such as the Seonghak jibyo (Essentials of Sage Learning, 1575) alongside concrete reform proposals on taxation, border defense, and village compacts meant to translate Confucian moral cultivation into institutional practice.
Legacy
The Four-Seven Debate between Yulgok's and T'oegye's positions split later Korean Neo-Confucianism into rival interpretive lineages that structured centuries of subsequent philosophical and factional alignment, and Yulgok's Seonghak jibyo remained, alongside T'oegye's Ten Diagrams, one of the two central texts of Joseon sage learning.