From the book opening:

If we could only get used to the idea that ghosts are perfectly harmless creatures, who are powerless to affect our well-being unless we assist them by giving way to our fears, we should enjoy the supernatural exceedingly, it seems to me. Coleridge, I think it was, was once asked by a lady if he believed in ghosts, and he replied, "No, madame; I have seen too many of them." Which is my case exactly. I have seen so many horrid visitants from other worlds that they hardly affect me at all, so far as the mere inspiration of terror is concerned.