The document discusses optical illusions, explaining that they are visual illusions caused when the brain's perception does not match physical reality. There are three main types of optical illusions - literal, physiological, and cognitive - which respectively involve distorted images, eye/brain overstimulation, and unconscious inferences. Optical illusions differ from hallucinations in that illusions involve an external stimulus being misperceived, while hallucinations involve perceiving something without an external cause, such as from drugs or mental illness.