Friedrich Spee's book is built as a series of questions with answers, in the manner of a legal treatise, and its power comes from the fact that it concedes almost everything to its opponents before dismantling them. Spee does not argue that there are no witches. He treats that as an open question and sets it aside. His argument is about the machinery.
Torture, he says, produces a confession. It does not produce the truth, because what it produces is whatever ends the pain, and what ends the pain is agreeing with the interrogator. An innocent woman under torture will confess. Having confessed, she will be asked who else was at the gathering, and she must name someone, because she has already agreed there was a gathering. Those she names will be arrested and tortured and will name others. There is no exit anywhere in the sequence. A defendant who denies is tortured for denying, a defendant who confesses is executed, and a defendant who retracts is tortured again for retracting.
Spee had the authority to say this because of where he had been. As a Jesuit priest in the German prosecutions he had heard the confessions of the condemned and walked with them to execution, and the book carries the flat, exhausted certainty of a man who has been told the truth in the last hour by people who had nothing left to gain by lying. He states plainly that he does not believe he ever accompanied a guilty one.