In his opening address, he called for his students to disprove political and cultural myths that could lead to movements like National Socialism.

Geyl was best known as a critic of the British historian Arnold J. Toynbee, who seemed to maintain that he had discovered "laws" of history that proved how civilisations rise and fall.

Finally, Geyl was opposed to Toynbee's apparent claim that Western civilisation was in terminal decline.

Geyl was noted for challenging the then-popular theory that the historical separation of the Dutch and the Flemings was a result of "natural" causes.

In accordance with his historical ideas, Geyl actively supported the Flemish movement, though not favouring Dutch-Flemish irredentism.

[3] Geyl's work has been criticised for not taking into account the unifying force of administrative and economic developments after the separation and for sometimes drawing artificial boundaries based on language alone; on the other hand, it has been praised for its refreshing approach to the Dutch Revolt, which was in marked opposition to the then-current nationally oriented, almost finalistic view on Dutch and Belgian history as represented by P.J.

Geyl accused William IV of Orange of using the uprising of the Doelisten (a group of Amsterdam burgers) against the ruling elite to seize power for himself in 1748.

Another revisionist claim made by Geyl was that the marriage of William of Orange (later stadtholder Willem II) to Mary Stuart was the main cause of the first Anglo-Dutch War in the 17th century.