I want to express my sincere gratitude to a number of persons who have made this work possible. First of all, my deepest gratitude goes to Dudley Shapere. His inspiring lectures at the University of Maryland were instrumental in instilling in me a serious interest in Kepler. Over the years he has been a genuine source of help, inspiration, and encouragement. My special thanks are due also to Frederick Suppe for his personal interest, encouragement, and valuable suggestions at various stages of this work. Stephen Brush read its draft in the initial stages and made many valuable suggestions; I am grateful to him for this and for all the help and encouragement he has given so generously.
I want to thank the late Francis Haber, Owen Gingerich, Eugene Helm, James Langford, and Curtis Wilson for reading the manuscript and giving their expert comments and advice.
It is well known that Kepler's original writings are very difficult to translate. I have found helpful the existing translations of Carola Baumgardt, William Donahue, A. M. Duncan, Owen Gingerich, Alexandre Koyré, Wolfgang Pauli, Charles Wallis, and Ann Wegner. I have modified or retranslated the relevant passages whenever it was necessary to bring out more clearly the meaning of the original Latin text. In this connection I am particularly grateful to Owen Gingerich