Witches are men or women with higher mind abilities. The basis of their power is that their brain chemistry gives them access to brain cells that are able to process, store, and/or release high amounts of energy, which they absorb from the world around them. Generally this energy is referred to as aether (not to be confused with The Aether).
The trouble is that if a witch were to overtax her advanced brain cells, it would begin to degrade her normal brain cells, thereby leaving her mind anywhere between brain dead and slightly crazed. Such is the affect of aether on the unprepared mind.