In the almost one hundred writings and more than one thousand letters included in this volume, Einstein is revealed yet again as the consummate puzzler of myriad scientific problems as well as the invested participant in social and political engagements. He continues to explore the light quantum, whose reality is confirmed by new experiments, and to attempt to formulate a unified theory of gravitation and electromagnetism. He travels to South America, where he lectures widely on relativity, rejoins the International Committee on Intellectual Cooperation, and supports the idea of a European union. Einstein has a fourteen-month romantic relationship with his secretary, Betty Neumann, which ends in October 1924.