Insanity

Insanity was a condition of mental instability in sentient beings. It was colloquially referred to as madness or going crazy.

The supercomputer Xoanon was inadvertently driven insane when the Fourth Doctor helped repair it. (TV: The Face of Evil)

A psychic locket showed Alex Hopkins the future, and also drove him insane. (TV: Fragments)

Dalek Caan became insane after he saw time itself. (TV: The Stolen Earth/Journey's End)

Giuseppe di Cattivo was driven insane after painting The Abomination. (TV: Mona Lisa's Revenge)