The term illusion is used to describe situations where we make mistakes and perceive the surrounding world incorrectly. But what is an illusion? Richard Gregory described illusions as ‘departures from reality’ and this fits with our everyday idea that illusions represent situations where what we perceive does not correspond to some physical characteristic of the particular scene. ‘Delusions about illusions’ considers illusions that have a physiological basis and those with a cognitive basis. It seems very likely that our perceptual systems have evolved to extract the invariant characteristics of the world—the things that don’t change—rather than requiring mechanisms to ‘correct’ for things that do change.