The German prelate Nicholas of Cusa (1401–64) belonged to a period of history that was rife with transitional cross-currents. Some scholars, such as C. Warren Hollister, call this period the Late Middle Ages; others, such as Paul O. Kristeller, refer to it simply as the Renaissance. Furthermore, some intellectual historians who take soundings in the fourteenth and the fifteenth centuries claim to descry ripples of modern scientific enquiry as these issue forth from Theodoric of Freiburg's experiment of 1304, when, using glass balls, he ascertained that a rainbow results from light's passing through a medium whereby it is both reflected and refracted. And these same historians point to William Ockham's philosophical nominalism and to his doubts about the validity of natural theology. By contrast, other intellectual historians choose to emphasize the continuity of the fourteenth and fifteenth centuries with the past, as instanced by the unceasing ecclesiastical disputes and by the ongoing vigorous reactions to the incursions of Islam into the West. Nicholas himself is caught up in these historical cross-currents: in the flow towards modernity and towards new ways of conceptualizing, as well as in the ebb towards the past and towards traditional patterns of thought.