remarkable 'debate' the orthodox take on the nature of reality: Truth is not external. Reality only exists in the mind, and as the Party exercises absolute control over the minds of the citizens of Oceania, the Party controls truth. O'Brien provides a horrifying illustration of this when he maintainssimultaneously the primitive view that the stars are bits of fire easily reachable by men, and the scientific one, that they are suns countless light years away. The point being made is that reality is infinitely tractile to the power of the Party. As O'Brien, the philosophical idealist, exultantly maintains in the face of the weakening counter-arguments of Winston Smith, the realist, the only objective of the state -- no longer chained to the outmoded vision of improving humanity -- is the everlasting increase in power.

Orwell presents a futuristic scenario in which "authority" exists in an extreme form that subjugates an entire population. It isn't the concept of authority itself that Orwell is questioning, but the perversion of it in totalitarian states. In Oceania (and evidently in the two other super-states, Eurasia and Eastasia, as well) a ruling group has taken absolute control and has abolished the right of people to question its authority. This is, to put it somewhat simplistically, the difference between a democracy and a dictatorship. In a free society, as has historically existed in Britain, the United States, and other democracies, the authority of government is legitimate because it comes, however imperfectly, from the people, rather than being an instrument to oppress the people, as it was in the Soviet Union and Nazi Germany, and as it is in the dystopia of 1984.

In 1984,it is the Party that questions the nature of reality, or rather, seeks to destroy the entire concept of reality as an objective thing which people can independently be aware of. It demands that the population accept "reality" as whatever the Party declares it to be, even if this conflicts with the evidence of people's senses and their memory. Even before O'Brien states this directly to Winston in the interrogation/torture sessions, we see the principle in action when Winston is attending a rally in Victory Square. The speaker suddenly announces that Oceania, contrary to what Winston remembers as the truth, is and has always been at war with Eastasia and in alliance with Eurasia. Like automatons, the people are supposed to follow this blindly because the Party has declared it. After Winston's arrest, when O'Brien holds up four fingers and tells him he's showing him five fingers, he's making explicit what Winston has already recognized as the principle behind the Party's mind-control of the population. Reality, in Oceania, becomes whatever the Party says it is.

Both of these passages come from Part One, Chapter 7, I read the book on-line, no page numbers available 

In 1984, the nature of authority and reality are shaped by the party.   

Reality 

"In the end the Party would announce that two and two made five, and you would have to believe it. It was inevitable that they should make that claim sooner or later: the logic of their position demanded it. Not merely the validity of experience, but the very existence of external reality was tacitly denied by their philosophy."

Authority

"The Party told you to reject the evidence of your eyes and ears. It was their final, most essential command. His heart sank as he thought of the enormous power arrayed against him, the ease with which any Party intellectual would overthrow him in debate, the subtle arguments which he would not be able to understand, much less answer. And yet he was in the right! They were wrong and he was right."