On Information and Belief in Casting Illusions

This book heavily features the idea that magic can be disbelieved but it was so terrible I couldn't finish it.

The Goblin's Henchman blog had a post today about the way phantasmal force and other illusion spells used to work. For anyone that doesn't know illusions would cause real damage and could even kill anyone who thought that an illusion was real. Once someone realized that whatever was hurting them was an illusional all the damage would disappear.

I don't know where exactly this idea came from but I have read some old pulp books where that is the way magic works. The most direct anolog is The Incompleat Enchanter  series written by L. Sprague de Camp and Fletcher Pratt in the 1950's that features a group of psychologists from Cleveland that are good at fencing and use their psychology research to travel to different dimensions and become wizards.  The third book in the series, Wall of Serpents, which is a complete piece of garbage, features a form of magic that is deadly to anyone who believes in magic luckily the swashbuckling psychologist has brought along his literal minded friend who can see through magic. Here is the part where the Finnish hero Lemminkainen explains it to another enchanter, "'No,' said Lemminkainen, 'for his art is the seeing eye that penetrates all magics, and if you challenge him, you have already lost, since he penetrated your disguise.'"

As the magic gets stronger in the book it isn't enough to know that the spell is an illusion you must also know from what the illusion was created. Here:

They gazed at the spectacle for a moment or two. It was fairly revolting, but the snakes made no movement to leave their position.
Suddenly Pete Brodsky said, "Hey! I got a idea."
"What is it?" asked Shea.
Brodsky jerked his thumb toward Vuohinen. "this gummy belongs to me, don't he?"
"Under the laws of this country, I believe that's right," said Shea, and "He is your serf,"said Lemminkainen.
"And he's on this magic lay in this joint?"
She said, "Why so he is , now that you mention it. He must have been the one who worked the rive of fire and the eagle."
Brodky reached a hand out and grabbed Vuohinen by the collar. "All right punk! What's the right name for them potato-water dreams out there?"
"Awk! said Vuohinen. "Never will I be a traitor ..."
"Bag your head on that stuff. Come across with the right dope, or I'll have shorty here let you have it." He pointed significantly to the sword that hung at Lemminkainen's side.
"Awk!" said Vuohinen again, as the hand twisted in his collar. "they are–made from lingonberries."
Walter Bayard said, "Why, so they are!" He walked across to the hissing, snarling barrier, reached out his hand, twisted the head of one of the serpents, and ate it. 

So less powerful magic can be dispelled by someone who is rational and skeptical and more powerful magic can be dispelled by a rational and skeptical person who has an understanding of how the spell was created.  Interestingly, wizards have a much harder time doing this making it similar to Conan's ability to sometimes negate magic through disbelief. Unfortunately, I can't find my book of Conan stories right now so I can't get a relevant quote.

You could take this idea that if you know what the illusion was made from you can see through it as opposed to just not believing in an illusion in order to see through it to make disbelieving illusions more interesting. 

This book is in the same series and I was able to slog through it. It still sucks though.