*This paper was presented at a conference on scientific concepts of time in humanistic and social perspectives organised by J.T. Fraser and held at the Rockefeller Study Center, Bellagio, Italy, in July 1981. I wish to thank Y. Elkana, Director of the Van Leer Jerusalem Foundation and M. Ron, Curator of the history of science collections at the Jewish National Library at the Hebrew University in Jerusalem, for making facilities available to me in researching and preparing this paper.