What does Winston mean by, "Freedom is the freedom to say that two plus two makes four. If that is granted, all else follows"?

In the
Ministry of Truth, whereworks, his job is to manipulate information constantly so that
everything that is disseminated unfailingly supports the Partys view of eventsa 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 Partys 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.