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This book was designed to explore as fully as possible the appropriateness of the phrase immutator mundi or transformer of the world, as applied by contemporaries to Frederick II of Hohenstaufen, to establish the relationship of his many-sided achievements to those of his Norman and Hohenstaufen antecedents; to describe the circle of associates who participated in his manifold activities; and, finally, to seek the origin and to trace the course of the unremitting hostility of contemporary popes to him and to his concept of empire. The author has critically examined and judiciously employed all available contemporary chronicles, letters, official documents, polemical writings, and all other pertinent materials that either directly or indirectly bear upon the subject. In addition, the book is in no wise concerned with the spiritual motivation of the priesthood.