An empiricist's grasp of facts and data enables him to anchor himself in reality, granting resistance to even the most potent illusions. At 4th level, an empiricist gains a +2 insight bonus on all Will saving throws against illusion spells or spell-like abilities that allow a save to disbelieve their effects. In addition, he can expend one use of inspiration as an immediate action to use his Intelligence bonus instead of his Wisdom bonus on all such saves for 1 round.
At 8th level, the empiricist's insight bonus increases to +4. At 16th level, he gains immunity to all illusion spells and spell-like abilities that allow a save to disbelieve their effects. This ability replaces swift alchemy.