Johannes Kepler, like so many other early modern scholars of natural philosophy, has often seemed full of contradictions, ironies, and rough edges, a Copernican who argued in print that the planets made music in counterpoint and who disputed not only about Galileo but about Lutheran theology. Aviva Rothman demonstrates convincingly in this study that all of Kepler's positions were, in his eyes, clear and consistent. She focuses especially here on several issues: Kepler's relationships with Lutherans and Catholics, his view on the state and on the political roles and responsibilities of the astronomer, the Galileo (and Copernican) controversies, and calendar reform.
Principles of harmony as presented by Boethius and other authors remained part of general education in Kepler's youth; accordingly, men of learning may have invoked them not just in reference to music or mathematics but also in discussing the social and political order and more. Modern readers have often found it difficult to assess how seriously they figured in these contexts. For Kepler, as Rothman shows, they were serious indeed. He had originally studied theology, and even though he devoted most of his professional life to reading the divine hand in nature rather than in scripture, these two tasks remained linked in his mind.
Kepler's first published work, the Mysterium Cosmographicum, presented his understanding of cosmic harmony. He argued that the Platonic solids, nested and centered on the sun, correlated with the distances between the planets. Further, geometric forms did not merely describe but genuinely represented the divinely imposed order of the world. He argued that scripture could be reconciled with Copernican theory, though both also convinced him that Calvinist positions on the Eucharist were preferable to the Lutheran formulation. His colleagues in Tübingen disagreed.
Kepler continued to believe that God spoke to humanity through nature, not only in the language of mathematics but also in more allegorical ways, as was necessary to reach out to people of diverse eras and levels of learning. As he noted from De Stella Nova (1606) through his correspondence in the 1620s, God might even use the language of astrology. Kepler extended his invocations of harmony, as a polyphonic resolution of discords into concords, from his communications with fellow astronomers, across confessional divides, to his hopes for the church. Kepler retained a vision of the church as the body of all the faithful, not merely those of a single confession, and hoped for unity. He supported Galileo and Copernicus both on their own merits, and as part of that goal of unity.
Kepler also had something of a public role as Rudolph II's mathematician. The customary expectation of astrological pronouncements involved political prognostications. Kepler avoided particulars, but emphasized the consistency between cosmic and political harmony, notably in his 1619 On the Harmony of the World. He supported calendar reform, which he distinguished from its original papal sponsors. Rothman compares Kepler's invocation of harmony in political thought with Bodin, and other contemporaries, who found the concept similarly compelling.
Kepler's efforts to persuade others of his positions were not uniformly successful. He failed to convince his fellow Lutherans either about ubiquity or about freedom of conscience, and was excommunicated. His effort to support Galileo in his Dissertatio was misread by his younger colleague Martin Horky, who sowed confusion when he went into print claiming Kepler as an anti-Copernican ally and citing the Dissertatio as evidence. Rothman contextualizes Kepler's positions on politics, conscience, and more, successful or not, among the writings and careers of such contemporaries as Bodin, Leibniz, Descartes, and others; she describes his position as a sort of cosmopolitanism, a tradition she connects in turn with twentieth-century figures such as Hannah Arendt. In so doing, she presents Kepler and his colleagues in a long line of scholars who also had public profiles, and who wrote about society and contemporary issues alongside and together with their more specialized disciplinary writings.
A more explicit narrative thread might have helped keep the reader oriented throughout. Nonetheless, the Kepler who emerges from these pages is clear, consistent, and admirable. Rothman has succeeded in presenting us with a fuller Kepler and, in the process, a much richer sense of early modern natural philosophers in their world.
