However, Lok-Ham Chan, a professor of history at the University of Washington, writes that Changchub Gyaltsen's aims were to recreate the old Tibetan Kingdom that existed during the Chinese Tang dynasty, to build "nationalist sentiment" amongst Tibetans, and to "remove all traces of Mongol suzerainty." Georges Dreyfus, a professor of religion at Williams College, writes that it was Changchub Gyaltsen who adopted the old administrative system of Songtsän Gampo (c. 605–649)—the first leader of the Tibetan Empire to establish Tibet as a strong power—by reinstating its legal code of punishments and administrative units. For example, instead of the 13 governorships established by the Mongol Sakya viceroy, Changchub Gyaltsen divided Central Tibet into districts (dzong) with district heads (dzong dpon) who had to conform to old rituals and wear clothing styles of old Imperial Tibet. Van Praag asserts that Changchub Gyaltsen's ambitions were to "restore to Tibet the glories of its Imperial Age" by reinstating secular administration, promoting "national culture and traditions," and installing a law code that survived into the 20th century.