Going nuts–or, rather, not going nuts–was the soul and centre of witchcraft, and this was how it worked. After a while, a witch, who almost always worked by herself in the tradition of witches, had a tendency to go… strange. Of course, it depended on the length of time and the strength of mind of the witch, but sooner or later they tended to get confused about things like right and wrong and good and bad and truth and consequences. That could be very dangerous. So witches had to keep one another normal, or at least what was normal for witches. It didn’t take very much: a tea party, a singalong, a stroll in the woods, and somehow everything balanced up, and they could look at advertisements for gingerbread cottages in the builder’s brochure without putting a deposit on one.
— Terry Pratchett, I Shall Wear Midnight