A writer can rarely claim with certainty that his treatment of a subject is the first time that the subject matter has been considered. This, however, is what I experienced with the topic dealt with in this volume. This is so, despite the fact that I had at my disposal no sources that my predecessors or contemporaries lacked. Volumes of halakhah, Jewish law, mostly of the responsa type, which I used are the same texts constantly studied by Torah scholars and rabbis ruling on halakhic questions. Modern scholars, especially historians, study these sources for the purposes of their own types of research and are thus also familiar with them. If my treatment of the selfsame volumes has led to unique results, this is because I made use of both approaches to learning: that of traditional scholarship and that of critical research. In accordance with these two perspectives of analysis, I trust that the book will be regarded as a contribution to both the history of society and the study of halakhah. Although the book itself is essentially historical, scholars may find in it a key to the understanding of some modern problems as well, in that the question of the application of halakhah to changing conditions of life has not ceased to intrigue us. To trace the hesitation that characterizes the adaptation of halakhah regarding the question of the Sabbath Gentile may well afford us a glimpse of the limits of the adaptability of halakhah in other fields as well.