INTRODUCTION TO VOLUME 5 XXXV pensed with quanta altogether, he gives up and no longer tries to find out whether quanta "really exist," as he puts it, but confines himself to "investi- gating as carefully as possible the consequences" of quantum theory, in order to explore the limits of its applicability.[10] Those efforts led to several impor- tant papers, not all of which were published, however. An example, well doc- umented in this volume, is a paper on the theory of residual rays, written as a reaction to experimental work by Heinrich Rubens and his collaborators. With his characteristic strong interest in experimental work, Einstein devel- oped a refined version of his theory of specific heats and molecular vibrations in an attempt to show that the measurements were misleading. He retracted the paper not only because of theoretical problems, but also because Rubens convinced him that the experimental results were correct.[11] Whereas Einstein's letters tell us nothing about the prehistory of special relativity, they do provide some background to the creation of general rela- tivity, albeit in a sketchy way. The crucial months of collaboration with Marcel Grossmann and the reasons behind their giving up the idea of general covariance remain in the dark. The later justifications and elaborations of their conclusions are somewhat better documented. But a deep insight into the thought processes that led Einstein along the road to the final version of general relativity that was published in the fall of 1915 is still lacking. IV Einstein's relationship with his fiancee and first wife has been touched on above, but it should be treated more explicitly in the context of his affair with his cousin Elsa Löwenthal which began in 1912. It seems that tensions between Einstein and his wife started to develop no later than 1909, when in a burst of venom Mileva attacked a woman she assumed to be a rival for Einstein's affection.[12] Quite possibly the situation was a harmless one, per- haps not. But the fact that three years later Einstein rekindled a friendship he had had with Elsa while both were children in southern Germany certainly reflects a serious deterioration in his relationship with Mileva. Several dis- paraging remarks about her person and their life together in his letters to Löwenthal testify to this development. In the letters from Einstein to Elsa-hers to him were destroyed at her request-which we present in this volume, she takes on both the character of lover and of much-needed confidante and refuge from the enervating exist- ence with Mileva. Still, matters were of course not as straightforward as they may seem in retrospect: torn between conflicting feelings and convinced that