This study is a superb example of historical scholarship. Martin Mulsow unearths an impressive range of mostly unknown German clandestine literature of the late seventeenth and early eighteenth centuries. His scrutiny of the various currents of intellectual life in early modern Germany is incredibly detailed, and at times somewhat dense and difficult to follow. The principal merit of this book is the way in which Mulsow manages to decode these different conversations, which in turn allows him to contextualise these often forgotten texts within the broader early Enlightenment.

One of Mulsow's main theses, which he corroborates throughout the book using a wealth of case studies, is to be seen in what he calls a ‘medium range theory’. Mulsow argues convincingly that ‘scholars [he does not mention Jonathan Israel, but it would be fair to assume that he is at the heart of this criticism] have overemphasised Spinozism as the crucial radicalising factor’ (p. 306) of the early modern radical Enlightenment. Instead he proposes that Jewish anti-Christian manuscripts played a major role in the development of early modern atheism. This attitude was related to different European religious and intellectual currents, of which Socinianism and Oriental studies are identified by Mulsow as the most important. According to Mulsow, Oriental studies ‘became the stage on which decisive intellectual battles were fought over the authority of the sacred when it was challenged by new experiences … and the questioning of traditions’ (p. 83). One of the underlying claims is, therefore, that the existing understanding of the early Enlightenment needs to be revised in view of this forgotten genre of writings and debates. Only in passing does Mulsow engage with the great names of the early German Enlightenment, such as Christian Thomasius. But he can demonstrate how we, in our interpretation of these pivotal figures, can gain further insight from looking at the ‘clandestine underground’ (p. 1) and the engagement with it by thinkers such as Thomasius. Mulsow shows how the result of this process was ‘to secularize scholarship and to remove metaphysical and theological vestiges from the sciences’ (p. 130). He also illustrates that the ‘clandestine underground’ was as much a creation of the orthodox powers who felt the need to react to these provocative vibes, as it was a deliberate creation of a small circle of freethinkers. The absence of Catholics in this study is quite striking and Mulsow reinforces the claim that ‘Catholic Germany … played almost no role in the early Enlightenment’ (p. 304).

What was going on in the ‘clandestine underground’ of early modern Germany should not be described as one existing network, but as a loose and often isolated endeavour of individual thinkers who challenged existing authorities. The borders of the intellectual landscape were more often than not explored by students who only half-heartedly took on the orthodox mainstream. Within the German university system, students developed new and challenging forms of ridicule, ambivalent speech and irony. These texts circulated in manuscripts and were passed on, which could mean that they gained a very different meaning in different contexts. Mulsow shows in detail, and with remarkable mastery of an impressive range of sources, how sometimes rather innocent mockery could lead to radical religious criticism and atheism. One key example in this study is the fate of De tribus impostoribus by Johann Joachim Müller, which started as a private joke among a few friends and colleagues in Protestant northern Germany and became one of the most notorious clandestine works advocating atheism. Freethinkers and libertines were inspired by the book, but for them only the content mattered; they had no idea of the rather innocent original context. ‘Atheism as a game and a joking experiment had become atheism as effect’ (p. 109).

Another important aspect which is convincingly highlighted in this study is the fact that the moderate and the radical Enlightenment were not two distinct movements, but were often interwoven and connected. The most important example in this respect is Halle with its innovative university and the vibrant circle of scholars and friends around Christian Thomasius. Nikolaus Hieronimus Gundling, pupil and friend of Thomasius, defended even Hobbes's indefensible state of nature. The university professor Gundling even went as far as to write an essay which aimed to set Hobbes free from atheism (Hobbes ab Atheismo liberates). Gundling was daring enough to explore the borders within the acceptable intellectual and theological setting at a relatively free place such as Halle, but he soon had to face harsh criticism from the orthodox Lutheran establishment. Gundling was not part of the clandestine underground, but a well-established and reputable professor at the prestigious university of Halle. However, he did not shy away from the intellectually forbidden and scandalous. He was thus part of the grey zone which formed between the clandestine and the moderate Enlightenment. It is the great merit of this study to have sharpened our focus on the multi-layered and often forgotten influences which in various ways provided the backdrop to the criticism of orthodoxy and thus a path to modernity.