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It is a summer afternoon in 1615. In the small south German town of Leonberg, Katharina Kepler, a 68-year-old widow, is at home. A messenger knocks on the door, summoning Frau Kepler to appear before the ducal governor, Lukas Einhorn.

Katharina follows Einhorn’s messenger uphill through a few cobbled streets in the August heat. Standing in the large reception room of the ducal governor’s house, she listens, shocked and confused, as she is accused of being a witch and using her magical powers to cause harm to her neighbours.

It would take six years for the trial of Katharina Kepler to conclude, during which time she would spend more than a year in prison wondering whether she would face torture, or even burning or beheading. But what happened next is surprising. Despite the alarmingly slim chances of success, Katharina’s family, and principally her son Johannes Kepler – one of the most famous astronomers of the period – decided to take on the accusers and attempt a defence of their mother.

No other public intellectual figure would ever involve himself in a similar role. At the height of his career, the Imperial Mathematician put his whole existence in Austria on hold. Kepler stored up his books, papers and instruments in boxes and moved his family to southern Germany, all the while keeping the affair as quiet as possible to ensure that the accusation did not result in a loss of his own reputation, too. It would be the fight of his life.

In 1615 almost everyone believed in the existence of witches and the devil: harm might spread through touch or commonly exchanged gifts – an apple, a cake or a drink. Witches could potentially be neighbours, family and friends or domestic employees, transported to a witches’ Sabbath for a mass gathering with the devils to celebrate a reverse mass. Sixteenth-century images and sensationalist writing familiarised this scary figure of the demonic witch as an old, envious hag with her broomstick, keen to attack fertility and madly dance at the Sabbath. This was not about mass delusion or inexplicable collective mania. Rather, it was accepted that anyone could be drawn into witchcraft – a truly terrifying idea that could easily corrupt social trust.

And so, as news of the accusations got out, fears began to attach themselves to Katharina more easily. At her trial in 1620, 21 local people would testify. Leonberg’s schoolmaster, who had once been Johannes’ school friend, complained that Katharina, who though not poor was illiterate, constantly pestered him to read out letters Johannes had sent to her from Prague. In his testimony he reported that Katharina seemed to magically enter his house demanding that he should write a letter to Johannes for her; eventually, he claimed, she tempted him to sip a drink that, he believed, led to his subsequent paralysis.

A woman named Dorothea Klebl, born and raised in Leonberg and a perfectly trustworthy witness, gave evidence that was both surprising as well as legally damning. Five years earlier, she said, she had employed a young local seamstress to carry out some needlework. Just before this time, this girl had worked for Katharina Kepler, who, she said, had once urged her to stay overnight. “Close to midnight,” the girl had reported to Klebl, Katharina had risen from her bed to “roam about” in the main room of her house. As the seamstress woke up, she asked: “Why do you roam about in the chamber instead of lying in bed?” Katharina replied: “Would it not please you to become a witch?”, promising the young girl “joy and debauchery beyond measure”.

The defence

How do you defend a witch? It is often assumed that Johannes Kepler, the defender of Copernicus’s sun-centered universe, the man who discovered that planets move in ellipses and who defined the three laws of planetary motion, simply met ‘irrational’ claims of witchcraft with Enlightenment scepticism. In fact, modern distinctions between the rational and irrational – religion and magic – obscure how knowledge about humans and nature mattered equally to him and many of his contemporaries.

So, if not a straightforward refutation of witchcraft, what then? To understand the genius of Kepler’s defence, we need to understand the prosecution. At this time in England, James I advocated trial by dunking (famously, drowning proved innocence). Nonetheless, the notion that even a witch deserved a ‘fair’ trial remained important.

That meant, for example, that an accused woman, whatever the crime, could not speak for herself – she required a man to speak on her behalf – and that women alone were not considered to be reliable witnesses. It also meant that evidence of witchcraft that occurred after an accusation had been made could not be included. For an accusation in Katharina’s home territory of Württemberg to be legal, a successful prosecution required not just the authority of the ducal governor, but also the duke himself, his Stuttgart chancellery and the professors of law at Tübingen University. Witness interrogations and the prosecution and defence cases were heard locally and then transcribed and delivered to the authorities for judgement.

Consequently, a successful prosecution depended on its meeting the requirements of the Imperial Law Code. By contrast, an effective defence required Kepler to discredit every single witness through legal reasoning. In addition, Kepler insisted that a particular procedure, developed in Roman Law, was to be used – meaning that all documents had to be presented in writing. This was a stroke of genius – it fully informed the defence about the evidence and, critically, enabled Kepler to mount a defence founded in the use of the tools of his intellectual life: textual scrutiny.

The fact-orientated inquiry of ‘things themselves’ was increasingly championed in all intellectual fields. Kepler’s education had nurtured his patience for puzzling over specifics. He had closely combed through literary and scholarly texts all his life, encouraged by the humanist devotion to meticulous philological analysis as well as a new Mannerist delight in keeping the mind flexible through enigmas, riddles, hidden jokes, intricate conceits and allegorical word play.

This had trained Kepler to attend to details and contradictions in the testimonies, a skill he had also honed in refuting scholarly opponents. The new taste for experiments and observations taught him how to assess who was doing the thinking, with what kind of authority and intentions, in a particular case, to produce what kind of data. If history and philology were needed to evaluate data presented by ancient writers, sociological context was needed to evaluate contemporary scientific arguments about singular natural phenomena, such as comets.

Kepler rigorously interrogated testimonies to establish which might be reliable. It mattered to any natural philosopher whether someone who observed, say, orbits, was female, socially marginal, self-interested and known for major moral shortcomings, or, by contrast, male, possessed of a solid social status and was long-experienced in particular areas of knowledge. Such insights from the human sciences and legal hermeneutics forged in antiquity had become just as important in natural history, and explain why leading minds took care to appear authoritative in the impression they made on others through observing etiquette and decorum.

In their actual writing, good natural philosophers then drew on legal methods to argue persuasively about who deserved to be trusted. They tried to end fruitless controversies about what was true or not in tricky matters by reaching a consensus about reasonable conclusions. These methods “bound science to scholarship and mathematics to letters”. This was why Kepler not only read the documentation in his mother’s case but also listened attentively to her in prison, as she described how it was normal to act in her world, how a Leonberg widow did widow-like things. Kepler did not descend from an elevated life of the mind to dirty details of a criminal trial. Years of arguing his case in science had prepared him to mount an exceptionally effective defence based on demonstrating that his mother’s reported behaviour was not witch-like but widow-like, on identifying who was a reliable witness and who was not, on clarifying the exact sequence of cause and effect – on, in other words, examining and testing every single item of the prosecution’s evidence.

Torture

It would take more than a year for the defence and prosecution cases to be heard and due process to be followed. But on 20 August 1621, despite Kepler’s best efforts, the ducal court’s advocate, Hieronymous Gabelkhover, in a tightly argued final accusation, left no doubt that Katharina had to be tortured. Barely any time remained for Kepler to compose his concluding defence. A legal ultimatum had been set. He had been able to plan for it in outline since May, but now needed to respond to the specific points of emphasis and particular arguments in Gabelkhover’s surprisingly severe accusation. Two local scribes simultaneously helped to copy Kepler’s furiously written draft into 64 neat pages. Kepler carefully went over this version, completing it with marginal glosses, additions and corrections. Desperate to end the case, he handed it in two days early.

And it worked. On 22 August, all the documents were sent at once to the Tübingen law faculty for its professors to now decide whether or not Katharina Kepler was to meet the torturer. In the event, Tübingen’s six professors of law worked quickly. On 10 September 1621, they sent a characteristically brief and diplomatic one- page document, fully in line with precedent. They held that much of the evidence was not sufficiently supported and, given Katharina’s old age, did not justify proper torture. Yet the woman could not simply be absolved either. A final attempt to “frighten the truth out of her” was needed. Katharina was to be led to the usual place of torture to face the executioner. He was to show his instruments and threaten to use them, although he was not allowed to actually touch or torture her. It would be Katharina Kepler’s final ordeal.

In the early morning of 28 September, having likely been warned by her son that this was a trial of nerves not flesh, Katharina was escorted from her tower gate prison to the town hall. There is no doubt that she felt this to be a dramatic moment. She was reported to have said: “I do not want to confess or admit anything at all. Even if you treat me whatever way you want, and tear one vein after the next out of my body, I would not know what to admit to.”

The governor, judges and executioner kept on threatening and reminding her of her deeds for some time, but she remained constant and entirely coherent. Finally, the guards were ordered to take Katharina back to her cell, where an anxious Johannes presumably awaited her. Six days later, his mother was unlocked from her iron chain and set free. She had spent 14 months in incarceration under the severest conditions. But her son had proven in law that she was not guilty of witchcraft.

Katharina Kepler died, aged 75, on 13 April 1622. No evidence survives about the final six months of her life. And Johannes Kepler never told anyone back in Linz the reason as to why he had temporarily moved back to Württemberg.

This epic tale has been transformed into an opera, Kepler’s Trial, to be performed on 28/29 October as part of the Cambridge Festival of Ideas. For more: festivalofideas.cam.ac.uk and keplers-trial.com.