Why must the prevailing mental condition be that of "controlled insanity" in order for the High to keep their places permanently atop the social ladder?

In George Orwell's 1984, the protagonist, Winston Smith, reads in Goldstein's book that in order for the "the High" to "keep their places permanently," the "prevailing mental condition must be controlled insanity." This is from a passage of the book explaining "DOUBLETHINK," which is the intellectual process by which followers of Big Brother can reconcile two contradictory beliefs.
Those who work for the Ministry of Truth, for example, believe completely in the history which they invent and rewrite, and at the same time disbelieve the history which they have erased. In psychology, this is known as a form of cognitive dissonance. It is a form of mental gymnastics designed to protect one's dominant world view. In simple terms, the mind suppresses its capacity for rational deduction in order to maintain the coherence and security of the aforementioned world view.
If insanity can be defined broadly as some kind of profound irrationality, then doublethink certainly qualifies as insane. But in 1984 it is an insanity which is necessary for the survival of Big Brother. Without this insanity, Big Brother would collapse under the weight of its own contradictions and lies. The insanity is "controlled" in the sense that it is re-enforced by relentless propaganda, the constant threat of violence, and ubiquitous surveillance and monitoring.