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

Guilana, the Unbound Doctor's therapist in a parallel universe, told Bernice Summerfield that those in her profession preferred not to use the word "insane". (AUDIO: Asking for a Friend)

Gideon Crane, believing himself to be the Doctor, asserted that "madness" was simply a matter of one person's reality not agreeing with another's own reality. In such cases, one or both of them was said to be mad, whereas two people experiencing a shared delusion, or both deceived, might well call themselves sane. (AUDIO: Minuet in Hell)

According to the Fourth Doctor, "There is no great genius without some touch of madness." (AUDIO: The Crowmarsh Experiment)

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

After falling into an abyss on Kastria, the version of Eldrad that was regenerated from the hand was driven mad in his solitude. He talked to the dust that surrounded him - the remains of long-dead Kastrians. (AUDIO: Eldrad Must Die!)

Liz Shaw asked Colin Dove if insane people were able to see the reservoir of evil. Dove told her that sensitives were able to. (HOMEVID: The Zero Imperative)

Object 1 (AUDIO: The Torchwood Archive) showed Alex Hopkins the future, leading him to shoot his teammates and himself. (TV: Fragments)

When Eric Klieg and the Brotherhood of Logicians attempted to awaken the Cybermen in a bid to obtain power, the Second Doctor extolled Klieg's genius and brilliance. When Klieg responded positively and longingly to the concept of world domination, the Doctor dropped the act and concluded: "Well now I know you're mad. I just wanted to make sure." (TV: The Tomb of the Cybermen)

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

Insane Daleks were imprisoned in the Dalek Asylum. This planet was destroyed by the Parliament of the Daleks when its force field was lowered by the Oswin Oswald. Several of the Dalek inmates were driven mad after surviving encounters with the Doctor. (TV: Asylum of the Daleks)

The Ninth Doctor declared the Dalek Emperor's army of Daleks bred from human remains to be insane, "driven mad by their own flesh". He claimed their hatred of their own existence made them deadlier than ever. (TV: The Parting of the Ways)

The Cybermen did not understand the concept. (AUDIO: Return of the Cybermen) Their own emotions were suppressed to prevent them from contemplating their own conversion, as this realisation would drive the being inside insane. (TV: The Invasion, The Age of Steel)

Vincent van Gogh suffered from insanity near the end of his life. (TV: The Pandorica Opens)

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

Jonah Bevan was driven mad after looking into the heart of a dark star. (TV: Adrift)