It is as if my life unfolded in 19-year cycles. After the first such period as a child and teenager in my parents' home, I spent the next 19 years (1910–1929) as a student, and then as a professor at German universities, admittedly also including 4 years as a soldier in the First World War. During the next 19 years (1929–1948), I was a professor at the Hebrew University in Jerusalem, in the land of Israel/Palestine under the British Mandate. Although I continued as professor after 1948, and as emeritus professor from 1959, the fourth 19-year period focused more on activities for the benefit of the State of Israel rather than strictly on my academic career.