The second historical 'knowledge' is the idea that the witch hunts were a medieval invention brought forth from the smelly dirty time where people had no sense of reality. Luckily we were saved from this nasty brutish existence by that wonderfully golden age the renaissance.
About how the witch hunts actually started I'll quote a little from his book, page 215. Information in [ ] brackets is supplied by me to explain things the author explained earlier.
"Almost throughout the Middle Ages - very generally until the thirteenth century, in some parts of Europe even to the fifteenth century - the accusatory form of criminal procedure obtained. That is to say, the legal battle was fought out not between society and the accused [as was the case in the witch hunts and heretic trials, where the Church was often the accuser and could employ torture to achieve 'confessions'. This was also the case when asking an accused to denounce others, something that wouldn't happen in trials brought forth by an individual.], but between the accused and a private person who accused him. In this respect there was no difference between a civil and a criminal case; in the latter as in the former the individual complainant was responsible for finding and producing proofs such as would convince the judge.
The accusatory procedure was derived from Roman law, and it retained all those features which had characterized it under the later Empire. By and large it favoured the accused rather than the accuser. The accuser was obliged to conduct the case himself, without the assistance of prosecuting counsel. Moreover, if he failed the convince the judge he was likely to suffer as heavy a penalty as would have been visited upon the accused if he had been convicted. This was known as the talion.
The intention behind the talion was simply to discourage malicious or frivolous accusations, but the effect was far more sweeping. How was the law to distinguish between a mere mistake and deliberate calumny? In practice it seldom distinguished; everyone knew that an unsuccessful complainant would almost certainly be penalized, whatever his motives. ...
Everything possible was done to impress the would-be accuser with the risks involved. When notifying the judge of the proposed action, the accuser had to give a written undertaking to provide proof and, if the proof were found inadequate, to submit to the penalty of the talion as a culminator. And that was not all: once the inscription had been accepted by the judge, the accuser could not withdraw without incurring the penalty of the talion."
Now, this does not mean people were never tried for maleficium (witchcraft) and punished. Nor does it mean the middle ages were an entirely lawful time, as lynching of suspected witches sometimes and maybe even often took place. There are a few isolated group trials of heretics and others performed during the middle ages by religious fanatics. But by and large the majority of witch hunts as considered in popular belief were, at best, late medieval and renaissance inventions.
Mr. Cohn concludes his work with the idea that later Ecclesiastes and bureaucrats were more willing to accept the truth behind accusations of witchcraft and evil doing in ways that those of earlier times were not. Allowing for miscarriages of justice where individuals were forced into confessions and then forced to denounce family and friends as being participants in an underground movement wherein the practitioners had denied Christ and begun worshiping the
Devil and performing malicious acts upon their neighbours.
At any rate, the book is fabulous - though it is fairly dry and scholarly - and the topic is fascinating.
Wikipedia's entry on the witch hunts is accurate and is a great jumping off point for learning more.