“Sanity was statistical. It was merely a question of learning to think as they thought.”
Source: 1984
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George Orwell 473
English author and journalist 1903–1950Related quotes
Many sources attribute this quote to Brown without giving a specific reference to her writings. The earliest located is the following variation from p. 47 of Musgrave Landing: Musings on the Writing Life by Susan Musgrave (1994), which Musgrave quotes as "Rita Mae Brown's warning": "If you become the kind of writer who calls forth heated emotional states, be careful. There are a lot of unbalanced people out there. The statistics on insanity are that one out of every four people is suffering from some form of mental illness. Think of your three best friends. If they're okay, then it's got to be you."
Disputed

“I salute your spunk, but I question your sanity,” Sam said.”
The Diviners
Variant: i salute your spunk, but question your sanity.
“Never question the sanity of a woman who can render you defenseless with a look.”
Source: By the Time You Read This, I'll Be Dead
"On Probability and Possibility"
The Lives of a Cell: Notes of a Biology Watcher (1974)
Context: Statistically the probability of any one of us being here is so small that you would think the mere fact of existence would keep us all in a contented dazzlement of surprise. We are alive against the stupendous odds of genetics, infinitely outnumbered by all the alternates who might, except for luck, be in our places.

Thought and Word, iv
The Note-Books of Samuel Butler (1912), Part VII - On the Making of Music, Pictures, and Books

"The Scientific Aspect of Monte Carlo Roulette" (1894)