You don't believe in ghosts, right? Well, neither do I. But how would you like to spend a night alone in a graveyard? I am subject to night fears, and I can tell you that I shouldn't like it at all. And yet I am perfectly well aware that fear of ghosts is contrary to science, reason, and religion. If I were sentenced to spend a night alone in a graveyard, I should know beforehand that no piece of evidence was going to transpire during the night that would do anything to raise the infinitesimal prior probability of the hypothesis that there are ghosts. I should already know that twigs were going to snap and the wind moan and that there would be half-seen movements in the darkness. And I should know that the inevitable occurrences of these things would be of no evidential value whatever. And yet, after I had been frog-marched into the graveyard, I should feel a thrill of fear every time one of these things happened. I could reason with myself: "I believe that the dead are in Heaven or Hell, or else that they sleep until the General Resurrection. And if my religion is an illusion, then some form of materialism is the correct metaphysic, and materialism is incompatible with the existence of ghosts. And if the Church and the materialists are both wrong and there are ghosts, what could be the harm in a ghost? What could such a poor wispy thing do to one?" And what would the value of this very cogent piece of reasoning be? None at all, at least in respect of allaying my fear of ghosts.

-- Peter van Inwagen