What does Winston mean by, "Freedom is the freedom to say that two plus two makes four. If that is granted, all else follows"?
When Winston says that "freedom is the freedom to say that two plus two makes four," he is asserting that truth exists independently of the Party's ideology. Crucially, this also asserts that the conditions of truth rest in part upon the external world; if Winston has the freedom to say that "two plus two makes four," he has the freedom to acknowledge existence independent of the Party's say-so.
In the Ministry of Truth, where Winston Smith works, his job is to manipulate information constantly so that everything that is disseminated unfailingly supports the Party’s view of events—a view that changes by the minute, not just by the day. Truth, in this frame of reference, becomes meaningless: there are no facts, just an endless supply of changes. Winston longs for some solid ground under his feet. Although he knows that mathematical and physical certainties cannot really be altered, his entire occupational responsibility makes him claim the opposite.
Winston is not saying that two and two do not make four. He wants the freedom to be able to oppose the Party’s total control of information. In part, he means that he wishes he could quit his job and not be complicit in supporting the unceasing distortion of information. Winston learns the hard way that there are facts: the main fact is that the Party will never allow anyone the opportunity to challenge its control. The most important word in his statement thus becomes the “if,” because the freedom to state a fact is not available to the people. People do not even have the intellectual freedom to state mathematical truths.
O’Brien is trying to force the Party’s own logic on Winston; more specifically, he is trying to control and reshape Winston’s reality—what he knows to be true—into whatever will suit the purpose of the Party. If Winston has the right to insist on his version of the truth—which is a truth that stands independent of ideology—then he can hold on some vestige of being human and determine at least that much of his life for himself. “You believe reality is something objective,” says O’Brien to Winston, but “reality …only in the mind of the Party…Whatever the Party holds to be truth is truth.” For Winston, if two plus two equals four, then reality is objective, and if this is true he can still seek objective truths and discount the double think of the Party.
In a society like Oceania, whatever they tell you is what you believe. Not the actual reality. If they tell you that 2+2=5, then it is 5 apparently, but really it is 4. If you have the freedom to believe that 2+2=4, then you can believe in almost anything.