The late Turrell V. Wylie, a former professor of the University of Washington, and Li Tieh-tseng argue that the reliability of the heavily censored History of Ming as a credible source on Sino-Tibetan relations is questionable, in the light of modern scholarship. Other historians also assert that these Ming titles were nominal and did not actually confer the authority that the earlier Yuan titles had. Van Praag writes that the "numerous economically motivated Tibetan missions to the Ming Court are referred to as 'tributary missions' in the Ming Shih." Van Praag writes that these "tributary missions" were simply prompted by China's need for horses from Tibet, since a viable horse market in Mongol lands was closed as a result of incessant conflict. Morris Rossabi also writes that "Tibet, which had extensive contacts with China during the Yuan, scarcely had diplomatic relations with the Ming."