"Well, in theory, if one believes something hard enough and strongly enough then it will become true. Wouldn't that be more of a psionic occurance rather than a magical one?"
"Whose theory is that? I don't think that's true at all. That's like saying that things will work out right because I want them to. And no, it's not psionic. How can I explain it? Try this: fire will burn you. Electricity will also burn you. If you put your hand on an uncovered wire, it will burn the skin off your hand. That doesn't mean that electricity and fire are the same thing, only that they have the same effect. If you throw water on the fire, it will probably cool and go out. Throw water on the electrical line, and you get an entirely different outcome.
"If I want to change the world psionically, that involves finding the strength within myself to bend reality to my will. Sure, that's possible; but that is drawing the power from me. Let's assume, though, that there is this other place, this world outside the world, where there is this different kind of power. If I can hook up that power to this world, I can use it to change the world, and the power doesn't come from me. With the psionics, it's like I'm running the system on my own internal battery; with magic, I just plugged it into a wall outlet, and got a lot of power from somewhere else.
"Of course, as I say, I don't care whether you believe there is such a power source or not. I've used it, and I'll use it again."
"You said that one must believe for the magic to occur, so does that mean that if someone's doing magic to me and I don't believe in it, then it won't happen?"
"It's more complicated than that. After all, you believe it won't happen, and he believes it will. You've got a resistance to magic from your unbelief, but it's not a complete immunity--his belief might be more powerful than yours (and what is unbelief but belief in reverse?). So maybe you can resist his magic, and maybe you can't. Probably it will affect you less than it affects someone else who does believe, but that doesn't mean it will never affect you at all.
"Also, you've got to be clear about what you mean by being hurt by magic. If I attempt to make you burst into flame, the fact that you don't believe is going to be a big factor in this, because I'm trying to hurt you with magic. On the other hand, I can instead cause a pillar of fire to fall from the sky and hit you. That's just as much magic when I create the fire, but for you it's not a question of whether you believe magic can cause you to burst into flame, but whether fire will burn you. Once the fire is there, it doesn't matter how it was created, it's still fire. I think most wizards and priests know this, so whatever protection you might have from unbelief is pretty quickly eliminated by a simple change in tactics."
--M. J. Young