Effects and magic items with this trait involve false sensory stimuli. Magic with the illusion trait creates false sensory stimuli. Sometimes illusions allow creatures a chance to disbelieve the spell, which lets the creature ignore the spell if it succeeds at doing so. This usually happens when a creature Seeks, Interacts, or otherwise spends actions to engage with the illusion, comparing the result of its Perception check (or another check or save the GM chooses) to the caster's spell DC. Mental illusions typically provide rules in the spell's description for disbelieving the effect (usually via a Will save).

If a creature engages with an illusion in a way that would prove it's not what it seems, the creature might know that an illusion is present, but it still can't ignore the illusion without successfully disbelieving it. Disbelieving a visual illusion makes it and those things it blocks seem hazy and indistinct, which might block vision enough to leave the other side concealed.