The aim was to examine Seongho Lee Ik`s learning and thought through considering 『Seonghosaseol(星湖僿說)』 「Cheonjimun(天地門)」. The writer could confirm some facts as a result of examining items and contents of 「Cheonjimun」.
His learning and thought can be considered the aspect that was confused the modern reason and the premodern knowledge. Seeing it with the whole items and the content composition in 「Cheonjimun」, he could be observed to have had the progressed reason more than any other characters in those days. Especially, the characteristics in modern reason are understood as the reason of possibility for the rotation of the earth in his astronomical view, as the recognition of excellence in western calender of escaping Chinese culture, and as the perspective of trying to understand the northeastern region in China differently from Chinese history while having great interest in the origin of Korean history. Also, the identification problem and the territorial consciousness on the Ulreungdo island are opinions that resulted from the modern position.
However, there are items and contents in which even the premodern reason and knowledge were slightly revealed yet. The reason on the traditional Confucian thought and the folklore is its typical case. Also, the understanding about Chinese civilization is relied upon materials without direct experience, thereby being thought to cannot help likely being premodern. Therefore, given relying upon the result of examining only 「Cheonjimun」, Seongho Lee Ik`s thought is possible to be carefully analyzed as saying of being high in modern aspect.