Disbelief

There was a brief discussion on the OD&D forums about Phantasmal Forces, so I thought I'd say something here about illusiona and disbelief. Some games, like The Fantasy Trip, have an explicit "roll to disbelieve", which in many cases actually dispels the illusion: you see a dragon, you say "I disbelieve!" and it vanishes. I have to admit, since I played both TFT and D&D back in the day, I have sometimes thought of magical illusions using this model; apparently, so have many others, as that seems to be the common interpretation even among D&D players.

But that's apparently not the original model for illusions in D&D. Consider the original Phantasmal Forces spell in Men & Magic:

"The creation of vivid illusions of nearly anything the user envisions (a projected mental image so to speak). As long as the caster concentrates on the spell, the illusion will continue unless touched by some living creature, so there is no limit on duration, per se. Damage caused to viewers of a Phantasmal Force will be real if the illusion is believed to be real. Range: 24"."

No saving throw is mentioned; unlike illusions in TFT's Wizard, which are designed to be playable in a no-GM environment, the GM calling for a saving throw immediately before describing an illusion would be a clue that perhaps things are not what they seem. Disbelief is only mentioned in the context of damage; disbelief doesn't cause the illusion to vanish, but touching it will. I'm seeing this as characters reaching out and having their hands pass through the insubstantial phantasm, after which it fades away. Ranged attacks won't dispel an illusion, unless you are using live cats as ammo; illusions can appear to dodge arrows, or an arrow will appear to stick in an illusory door when in reality it has fallen to the ground.

There's no mention in the spell description of a restriction to one sense, like sight or sound. I'd allow features that don't rely on substantiality, like heat, cold, even wind and moisture. In fact, the comment about damage is meaningless in combination with the insubstantiality restriction unless we accept the possibility of attacks that don't involve physical contact, like dragon's breath. Illusions of swordsmen or archers -- or, for that matter, pits -- are only useful in scaring or misdirecting enemies; fire, mist or gas, on the other hand, is insubstantial by nature, so illusory fire can burn, and poisonous clouds can suffocate (and instantly kill, if they are believed to be poison.)

So the real question in handling Phantasmal Forces is: what constitutes disbelief? I'd stick with the "no saving throw" approach; after all, if you can reach out and touch a phantasm, that dispels it automatically, with no roll required, so all you have to do is be close enough and brave enough to do that. For insubstantial attacks like fire or poisonous clouds, disbelief is automatic if the player can cite a reason for the disbelief, like noticing that your clothes are intact. Again, this doesn't cause the illusion to vanish, but it eliminates damage from the illusion. I'd give NPCs and monsters a morale bonus when ordered to attack a dragon they don't believe is real.

I'd handle actual illusionists a little different, because I would want to make their art a completely different kind of magic, with its own rules. But that's a matter for another post.