Fear

Fear was an emotion.

Some beings could feed on fear, like the entity on Phobos (AUDIO: Phobos) and the Fearmonger. (AUDIO: The Fearmonger)

The Keller Machine fed off the fears of its victims. They included Arthur Linwood, who was afraid of rats; Kettering, who was afraid of drowning; the Third Doctor, who was afraid of fire; Alcott, who was afraid of the Chinese; and, who was afraid of the Doctor. (TV: The Mind of Evil)

Permians hunted by inducing a state of fear in the minds of their prey. (AUDIO: The Land of the Dead)

The First Doctor noted fear as an example of emotions to the Cyberman leader Krail, questioning "Have you no emotions, sir?" (TV: The Tenth Planet)

The Sixth Doctor listed vertigo with agoraphobia and xenophobia as fears which, when mixed together, would form "naked fear". (PROSE: The Ultimate Evil)

Confronting the Daleks, the Ninth Doctor voiced his belief that, despite their purging of emotions, one little spark of fear remained in their DNA. He goaded to them that that spark of fear "burn[ed]" when they faced him. (TV: The Parting of the Ways)

Addressing humankind shortly before the Battle of Canary Wharf, Cyber-Leader One assured the people of Earth that they need not fear as the emotion would be removed upon their conversion into Cybermen. (TV: Doomsday)

George psychically deposited all his fears in his doll's house, which manifested as peg dolls. (TV: Night Terrors)

On the Minotaur's prison ship were pictures of those who had fed it and what their fear was. They included Novice Prin, who was afraid of sabrewolves; Lady Silver-Tear, who was afraid of Daleks; Royston Luke Gold, who was afraid of Plymouth; Tim Heath, who was afraid of having his picture taken and Tim Nelson, who was afraid of balloons. (TV: The God Complex)

According to Miss Quill, the "first fear" was a fear that "everyone" went back to and could not face. The Lorr Ballon claimed that this was just cowardice. Quill claimed that a soldier without fear was inefficient, and was one that won battles, but lost wars. (TV: The Metaphysical Engine, or What Quill Did)