I wish to express my gratitude to several scholars who have had considerable share in the production of this book. With out the repeated and earnest solicitation and the encourage ment received from Professor Morris Jastrow, Jr., of the University of Pennsylvania, this book would not have been written. Its appearance in English is due to Professor B. J. Vos of the Johns Hopkins University, who, in view of his own deep interest in the subject, was especially qualified to under take the translation. The first eleven chapters — also pub lished in Dutch — have been carefully revised by Professor B. Symons of the University of Groningen, who has read the proof sheets with the keen eye of the specialist, and whose numerous suggestions have frequently proved of value in con trolling and correcting my own views.