Generally, any false opinion that a person persists in may be called a delusion. In the philosophy of perception, delusions such as hallucinations are sometimes distinguished from more everyday illusions, such as the bent appearance of a stick in water, or a mirage. Mirages and standard illusions are public and repeatable. They are cases in which a certain kind of stimulus naturally gives rise to an incorrect interpretation. A delusion, by contrast, is thought of as a private perceptual derangement.