How does Winston's reminiscing about the past trigger his temptation to rebel against the Party in 1984?
Winston's memories of the past demonstrate his knowledge that the world was not always like it is under the Party. He remembers, albeit vaguely, his mother and father as well as his baby sister, and their memories suggest, in a way, that he has not completely absorbed Party ideology. Certainly his dreams seem to occur simultaneously with the beginning of his diary, and may well have spurred him to think more critically about the Party. However, the important point seems to be not so much that his dreams spark his rebelliousness in some way, but that, in his world, his dreams and reminiscences are themselves acts of rebellion, in a word, "thoughtcrime." We see how dangerous thoughtcrime can be, even in one's dreams, when we learn that Parsons has been apprehended by the Thought Police for saying "Down with Big Brother" in his sleep. It is his own daughter who turned him in. So internal rebellion, even unconsciously, is significant and dangerous in its own right.