An entity attempted to punish the Fourth Doctor for all the fear he caused to people during his travels.
The entity pulled the Doctor's consciousness out of the TARDIS and made him experience several events that should have caused him fear: falling from a great height, being dismembered by a toothy creature, growing old and feeble, and being locked in a padded cell.
When the Doctor showed little fear, the entity angrily confronted him. It tried to enter his mind to find out what he feared. It eventually learned that the Doctor was afraid of losing his mind. However, the Doctor convinced the entity that since all this was in the entity's mind, if it condemned the Doctor to eternal insanity, it would condemn itself to eternal insanity as well. In the entity's confusion, the Doctor escaped. (PROSE: The Fear)