Sheer curiosity led me to read more and more of the old literature on probability. These explorations resulted in a number of papers during the 1980s, and at a certain point it occurred to me that I should try to write down the results of my efforts in a systematic way. No one had pursued the background of modern probability in any detail, so that I felt free to let my own particular interests act as my guide. As a result, the emphasis here is on foundational questions.

A historical–philosophical colleague may find the references to secondary literature few in number. This is due to the fact that literature on the development of modern probability is truly scarce, and it is also due to my insistence on consulting all the primary sources in the first place. I also wanted to combine a useful bibliography of primary sources with manageable length. There should still be enough indications to allow the uninitiated to begin reading the secondary literature.

I am indebted to several institutions for support during the period of writing the book. Most important, a fellowship of the Academy of Finland has secured the continuity of my researches. In 1982, I had the good fortune of being invited to join the project ‘Probabilistic Revolution’ at the Zentrum für interdisziplinäre Forschung at Universitä Bielefeld.