Dr. Shaw defines fear as a strong desire to escape from perceived danger to which the autonomic nervous system responds. As an example of one way to measure the extent of the fear response, Dr. Shaw shows a film of Dr. Charles Eriksen throwing a live snake at a woman to record her galvanic skin response and heart rate. Dr. Eriksen later demonstrates that familiarity and experience can be used to overcome a fear. Dr. Shaw describes the difference between fear and caution and explains that fears are more likely to become pathological than other emotions. Phobias are irrational fears associated with specific objects, usually associated with an intense or unusual experience in childhood. Examples of typical phobias include acrophobia, claustrophobia, xylophobia, dromophobia, agoraphobia, monophobia, zoophobia, ailurophobia, topophobia, and myctophobia. In an experiment to learn if a person could be conditioned to fear, J. B. Watson and Rosalie Rayner taught a child to fear a harmless object.

The brain perceives not only what is seen by the eyes, but also processes experience and expectations to sometimes perceive not exactly what was seen. Magic tricks depend on the perception of the eye. Optical illusions and moving picture frames show that the eye can be tricked. The ability to detect movement is relative, but eyes can determine distance by working together.