Short answers, pulled from the story.
Pieter Geyl (the 15th of December 1887 - the 31st of December 1966) was a Dutch historian based first at the University of London and later the University of Utrecht. He was known for his revisionist thesis that the Dutch and Flemish peoples shared a common "Greater Netherlands" history, and for his sustained public debates with the British historian Arnold J. Toynbee.
Geyl was taken hostage by the SD, the Security Service of the SS, in October 1940. The Germans claimed it was retaliation for the alleged mistreatment of Germans interned in the Dutch East Indies. He spent thirteen months at Buchenwald, and was subsequently held at Kamp Sint-Michielsgestel until his release on medical grounds in February 1944.
Geyl argued that the separation of the Dutch and the Flemish during the Eighty Years' War was not caused by cultural, religious, or political differences but by geography. The rebels succeeded in the north because lakes, bogs, and rivers hindered the Spanish Army; the flat plains of the south favoured Spain. He developed this thesis in his major work De Geschiedenis van de Nederlandse Stam, published between 1930 and 1959.
Geyl accused Toynbee of using evidence selectively to support predetermined conclusions, of relying on a "challenge and response" theory too vague to have explanatory power, and of claiming without sufficient basis that Western civilisation was in terminal decline. The two men debated publicly on radio and in print, and co-authored a discussion published in 1948.
Napoleon: voor en tegen in de Franse geschiedschrijving, published in 1946 and translated into English by Yale University Press in 1948, traces how French historians across different periods portrayed Napoleon as either a destructive Corsican adventurer or a patriotic bringer of prosperity. Geyl used the book to argue that all historical writing is shaped by the concerns of the historian's own present, making every account provisional.
Geyl taught at Stedelijk Gymnasium Schiedam before becoming professor of Dutch history at the University of London in 1919, a post he held until 1935. He then returned to the Netherlands as professor at the University of Utrecht, where he became chair of history in 1945. He was also a member of the Royal Netherlands Academy of Arts and Sciences, first as a correspondent from 1928 and then as a full member from 1946.