In the evidence that supports The Rabbit of Usk is an association between idea and witchcraft. This isn't fantastical, it's not myth, we're not talking about people flying around on friendly Dragons. If a person believes a spell has been cast upon them, it will dog their mind. If the spell caster has surreptitiously introduced a medicinal compound into his or her victim's daily gruel, odds are the physical effects upon the victim will enhance the collection of emotions the victim endures while under the spell. A visit to the doctor, who are generally considered good witches, is as much comforting as anything else, and then should the diagnosis be accurate, the prescribed medicine correct, the victim or patient improves.
The other area which I'd claim supports The Rabbit of Usk has to do with ego and
the importance to the ego of feeling special. The less special a person feels
the more likely it is that a person will believe things and see in things
qualities those things probably do not actually possess. Sharing those beliefs
with others, however absurd those beliefs might be to outsiders, proceeds to
confirm the belief by demonstrating the belief's existence through those central
portals of the social, which are as much stories as they are anything else and
yet it's those shared beliefs that unable us to form both disastrous and more
sobering cohesive units. I could go on to describe the Blood Oaths of the Mau
Mau and the Drums of the Acholi, but I'll not go looking at the older societies
to demonstrate how stubborn our hero can be.