Belief as Reality?

In the novel 1984, Winston is tortured to the point of betrayal of his moral principles, his convictions, and his love: "In the face of pain, there are no friendships, no alliances." At the end of the book, he says he truly believes that "The past was alterable". This view is Platonic: that our reality depends on our thoughts, and not upon some exterior, material world. Wht the Party did, throughout this whole book, was attempt to overstep the bounds of the material world: to change facts in people's minds. It did so successfully because it was able to manipulate the conceptions and ideas of the mind without having to change material facts. In this way, the term "Thought Police" fits very well- it polices our thoughts and thinks of them as crime (thoughtcrime) rather than our actions. Winston, in accepting the principles of the Party, such as doublethink, distanced himself from a material conception of the world. In our society, we might refer to Winston's Party-inspired ideas as his "denial" or "delusion". It's interesting that the Party thought the exact same thing of materialistic, non-Platonic Winston. I do disagree with the mentally-leaning inclinations of the Party's ideology- the distancing of material from fact. The argument extended by Winston in 1984 - that assumption of any existence is fallacious- disagreed with me. I thought that the acceptance of thought introduced by O'Brien in Part III of the book is just as fallacious, because of the assumption inherent in the acceptance of said thought.