Orwell knew...

George Orwell, 1984:

"You are a slow learner, Winston," said O'Brien gently.

"How can I help it?" he blubbered. "How can I help seeing what is in front of my eyes? Two and two are four."

"Sometimes, Winston. Sometimes they are five. Sometimes they are three. Sometimes they are all of them at once. You must try harder. It is not easy to become sane."