What is the difference between "Negates" and "Disbelief" saving throws?

Question

The PRD describes several types of saving throw, including "Negates" and "Disbelief":

Negates: The spell has no effect on a subject that makes a successful saving throw.

Disbelief: A successful save lets the subject ignore the spell's effect.

The only semantic difference between "you can ignore the spell effect" and "the spell has no effect on you" I can figure out is that the first one is voluntary (so you can choose not to disbelive). But the PRD says you actually can forego a saving throw (it includes the "Negates" type, doesn't it):

Voluntarily Giving up a Saving Throw: A creature can voluntarily forego a saving throw and willingly accept a spell's result.

I guess you "disbelieve" illusions and "negate" any other effects, but it is nothing but fluff. What is the mechanical difference between them?

Answer 1

Figments aren't "negated"

So, a common example of a disbelief spell is Silent Image, a figment. When a character successfully saves against Silent Image, the Image is not 'gone', it "remains as a translucent outline". This leaves casters with the ability to still USE that Silent Image, for purposes that don't require the target to "believe" the image, such as passing messages.

Answer 2

For your character in particular, disbelief and negates are mechanically identical.

That is not true when you consider other party members. Consider Silent Image:

Silent Image has a will save (disbelief) if interacted with. This means that, if you pass the save, you know it is an image, and the spell continues producing its effect.
If it were will save (negates) - your save would counter the effect, and the image would be gone for everyone.

And this is why, as you pointed out, that you disbelieve illusions. Illusions aren't tied to you as the target - they generically apply to everyone who can see them, so one person realizing it's an illusion does not cause some hivemind effect such that everyone knows.

Answer 3

Disbelief only applies to illusions. As the text notes (under "Illusions") targets can't "recognize it as illusory until they study it carefully or interact with it in some fashion." Only after that interaction do you get the saving throw at all. An obvious fake requires no save, while a successful save lets you try to instill doubt in others.

You can refuse to believe in a fireball, but if it hits you, it's still going to leave a mark. A negation means the spell is effective, but through skill or luck, you avoid any damage (or other effect).

It's two different ways to save, one for illusions, one for everything else. They aren't equivalent. That's why they're listed separately.