First, at the first stage, Kim Yong-duck understood Korean History from the standpoint of the Stagnation Theory
Second, in early 1960's Kim Yong-duck's standpoint changed from the Stagnation Theory to the Internecine Development Theory.
Third, in mid-1960's Kim Yong-duck's standpoint to understand the late Choson dynasty as the Transition Period to Modernity became clear.
Fourth, in mid-1970's Kim Yong-duck became critical of the opinion to divide Choson Dynasty into two parts by Japanese Invasion War at the end of 16th century. Instead, he began to pay attention to social changes from early 16th century.
Firth, Kim Yong-duck's Internecine Development Theory of Korean History is deeply related to Korean Nationalism.
Sixth, Kim Yong-duck's study of Sirhak(Practical Learning) made it possible to understand Sirhak(Practical Learning) from standpoint of the Internecine Development since 16th century.