My favorite quote:
Sanity is not statistical.
- From Nineteen Eighty-Four by George Orwell.
What it roughly means is this:
Things are objectively true or false. Things don't become true because everybody believes them, or false because no one believes them. Winston(the protagonist) insisted upon the right to assert that two plus two is four, even if everyone else is saying that it is something else. In other words, even if millions of people believe something is false and only one believes it is true, that one person is not necessarily crazy.