Two earlier books were retroactively accepted as a PhD thesis and habilitation at the University of Münster in 1966, qualifying him for a university professorship. In 1968/1969, he briefly served as a lecturer at Theodor Adorno's former chair at the University of Frankfurt and then was appointed full professor of sociology at the newly founded University of Bielefeld, Germany (until 1993). He continued to publish after his retirement, when he finally found the time to complete his magnum opus, "Die Gesellschaft der Gesellschaft" (literally, "The Society of Society"), which was published in 1997, and translated subsequently in English, under the title "Theory of Society" (volume I in 2012 and volume II in 2013). This work described segmented societies where territory is a dividing line.