Sadness

Sadness was an emotion.

The Vardy were programmed to keep the colonists of the human colony planet happy at all times. When one of the colonists died of natural causes, her friends and family began to grieve. The Vardy, reading grief as the opposite of happiness, treated it like a virus and began killing the colonists as to them, eliminating sadness meant eliminating sad people. (TV: Smile)

When the Enigma loomed over Gallifrey, the War Doctor suspected that its otherwise alien mind had found a kinship and understanding with the great sadness it sensed in Leela, using the emotion as a form of first contact. (AUDIO: The Enigma Dimension)