Posession: Do you believe? "The Exorcist" was based on an actual exorcism...?

I have no doubt that it is based on real events where people got together and exorcised a woman. But that doesn't prove that the exorcism actually did anything, that there were demons to exorcise, or that any of the underlying beliefs are true. Remember, they burned witches at the stake, and no one uses that as proof that witches exist.