In 1615, in the small Württemberg town of Leonberg, an elderly widow named Katharina was accused of witchcraft. Her surname, Kepler, is chiefly familiar to us today because of her son: Johannes Kepler (1571–630), imperial mathematician, defender of the Copernican system, and author of the laws of planetary motion. Yet the trial of Katharina Kepler (1547–1622), née Guldenmann, is not merely an episode in the private life of a great scientist. Her trial forms a historical chapter of her own, which reveals how, in the early modern period, the conflict between different interpretations of bodily signs was resolved. Symptoms that medical reasoning might regard as ambiguous or open to several causal explanations were transformed into evidence of intentional poisoning or witchcraft. Rumors, the “bewitched touch,” and the illnesses that reportedly followed encounters with her became, in the eyes of magistrates and hostile neighbours, evidence of a crime; evidence that Johannes Kepler’s defence sought to challenge by restoring the distinctions between symptoms, causes, and criminal responsibility.

Historian Ulinka Rublack’s reconstruction of the case has shown how misleading it is to reduce Katharina to the conventional image of the “witch-like” old woman.[1] Earlier accounts often presented her as a hideous woman, quarrelsome, suspicious, and almost predestined to be accused. Katharina instead belongs to the social world of early seventeenth-century Württemberg: she was a widowed mother, an ageing woman of modest resources, and a member of a small Lutheran community. In this sense, her trial offers a window onto social life in southern Germany on the eve of and during the opening years of the Thirty Years’ War (1618–1648).

The accusation of witchcraft and the ensuing legal proceedings unfolded, amid interruptions and reversals, between 1615 and 1621; the formal criminal trial began with Katharina’s imprisonment in 1620. She was officially charged with bewitching two women, Ursula Reinbold and Burga Haller’s daughter, by causing illness through magical means, allegedly by touch and by administering harmful drinks.

Katharina was almost seventy when the accusations began. She had managed an inn and raised her children largely without the support of her long-absent husband Heinrich, a mercenary soldier who had spent years away from the family and died before the accusation unfolded. Katharina was not a learned physician, nor should she be imagined as a professional healer, as there is no firm evidence that she made her living from healing.[2] Like many women in her world, however, she knew how to prepare household remedies and mixed herbal drinks, sometimes offering help to others. This practical knowledge could be useful, but it could also become dangerous: a failed remedy or an illness following an encounter could be transformed into suspicion of witchcraft.

Suspicion depended on the interpretation of bodily signs. As shown by Ian Maclean,[3] during the early modern period, learned medicine regarded symptoms as effects from which hidden causes had to be deduced. Yet since the same physical sign could stem from different causes, it acquired greater evidentiary weight only when considered alongside other converging signs. In Katharina’s case, however, pain said to follow physical contact or illness experienced after consuming a drink were used to support a specific legal argument. The question concerned not the symptoms, whether real or not, but what causal and criminal conclusions could legitimately be drawn from them. In court, bodily examination mattered because sorcery was expected to leave physical traces.

The accusation of witchcraft against Katharina first took shape in the conflict with Ursula Reinbold, the wife of a Leonberg glazier. Their relationship worsened after disputes over payment and, more importantly, after Ursula came to believe that Katharina had harmed her through a drink or a malicious charm. On 15 August 1615, the quarrel took a dramatic turn. During a festive visit to Leonberg by the Duke of Württemberg, Ursula’s brother Urban Kräutlein, a court barber, and the local magistrate Lutherus Einhorn subjected Katharina to an irregular interrogation. Urban threatened her with a sword and demanded that she confess to bewitching Ursula. Urban even pushed Ursula against Katharina, as if bodily contact with the supposed witch might have the power to undo the spell. Magical harm was indeed seen as transmissible and concrete; it could pass through touch, food, or drink and produce noticeable bodily effects.[4]

Katharina, however, refused to admit any wrongdoing and the episode initially had no formal legal consequence. But she did not forget the humiliation. With the support of her son Christoph Kepler and other relatives, she tried to sue Ursula for defamation. The move was bold, perhaps necessary, but it also pushed the conflict further into the courts.

Soon afterwards, another person claimed to have been bewitched by Katharina. Burga Haller’s young daughter complained of severe pain in her arm after Katharina had brushed against her in the street. Her parents sought compensation. What might otherwise have remained an unexplained ailment acquired evidential force against the previous episode. Katharina was summoned by Einhorn and, according to the records, she tried imprudently to win him over with the promise of a silver cup. The magistrate reported the matter to his superiors, the judges of the Stuttgart court and, on 24 October 1616, the authorities ordered that Katharina be taken into custody and examined on suspicion of witchcraft.

This measure did not yet lead to the prolonged detention that began in August 1620, but the dispute had now moved decisively from private accusation into a formal criminal process. This is the moment when Johannes Kepler enters the story.

He was then working in Linz and was at the height of his career. By the time of his appointment in Linz, Johannes Kepler had already published Astronomia nova (1609), a work that laid the foundations for modern astronomy.[5] Ten years later, while in Linz, Kepler had also published Harmonice mundi (1619), in which–in keeping with a long-established line of Pythagorean thought–he interpreted astronomical and astrological phenomena in terms of musical ratios and formulated his third law of planetary motion.[6] Advanced as they were, these achievements coexisted with other beliefs, which included the practice of witchcraft.[7] His decision to intervene in defence of his mother was not a proto-Enlightenment rejection of witchcraft, but a careful legal battle against unreliable accusations.[8] The accusation also became a family tragedy.[9] While some relatives supported Katharina, the accusation also divided her family members, with Johannes’ siblings divided over the opportunity to care for and defend their mother.

In 1620, the crisis deepened. Katharina was arrested at the home of her daughter Margaretha in Heumaden and taken into custody, first in Stuttgart, then in Leonberg, and finally at the court in Güglingen. There, the charges were reduced and reformulated, but the conditions of her detention remained harsh. Kepler intervened again, requesting better prison conditions and asking that the remainder of the proceedings to be conducted by means of written memoranda.[10] Both requests were accepted. He also appealed to the Faculty of Law at the University of Tübingen, where his old friend Christoph Besold taught.

Katharina, by then in her early seventies, was chained to a stone floor and watched by guards. The physical suffering inflicted on Katharina stemmed from the fact that, during the legal process, the evidence reported by the alleged victims—pain upon contact, discomfort after drinking—had been relocated from medicine to the judiciary and interpreted as signs of intentional harm. The affected body had become a body of evidence.

Kepler’s defence sought precisely to separate these two kinds of evidence: bodily symptoms from the evidentiary weight attributed to them. By isolating each alleged symptom and considering alternative natural or psychological causes, Kepler exposed contradictions in the testimony and showed that illnesses attributed to witchcraft could be explained by ordinary causes, especially bodily weakness or imagination.[11] Kepler insisted on procedural regularity. Against an evidentiary process in which symptoms had become accusations, he tried to restore proportionality between sign and cause.

The final ordeal came in September 1621. The Tübingen jurists recommended the territio, as provided for by the Constitutio Criminalis Carolina of 1532: the formal threat and display of the instruments of torture without their immediate physical application.[12] Although regarded as the least severe available coercive measure, it was no mild experience for an elderly prisoner already fatigued by prolonged confinement. On 27 September, Katharina faced the instruments but refused to confess any wrongdoing. On 3 October, the duke ordered her acquittal. Johannes Kepler’s defence had prevailed: bodily symptoms may have multiple causes and criminal intent had to be proved rather than inferred. A few days later Katerina was freed, but she eventually died a few months after, on 13 April 1622.

The case of Katharina Kepler forms part of the broader history of European witch persecutions, which intensified during the early modern period. For the history of medicine and the body, her trial is especially instructive. It shows how fragile the boundaries between healing and harming were in the early modern period, and how these ambiguities disproportionately affected women practitioners and non-professional healers. Above all, however, it reveals that medicine was not merely a response to social beliefs but an active force in shaping them, as social proximity itself came to be interpreted through contemporary notions of contamination and contagion.