We can say that Winston loses an essential part of his humanity, his soul, by the end of the novel when, after a life-time of secret rebellion, he succumbs completely to the mind control exerted by the Party. Brutalised and tortured, subjected to the most relentless interrogation, he loses his own individuality, the sense of his own self, his own needs, desires and hopes. He betrays his lover Julia, loses the ability to think for himself, to question, to challenge, to act of his own free will, to remember his past, and to have any emotion other than love for Big Brother and hatred for Big Brother’s enemies.
But it was all right, everything was all right, the struggle was finished. He had won the victory over himself. He loved Big Brother. (Part 3, chapter 6)
The above quote, from the very end of the book, shows how complete the Party’s dominion over Winston becomes. The defeat of Winston, the individual rebel, is ironically presented here as a ‘victory’, because Winston is now thinking and feeling exactly as the Party wants him to think and feel; this is the viewpoint of the totalitarian state, from which Winston is now indistinguishable.
Yet it may be questioned whether Winston’s final immolation in the Party does indeed represent the total extinction of his humanity. After all, he is still able to think and feel and function at the end, even if to no other purpose than to serve the Party. It might be more accurate to say that his humanity has become distorted, as his natural feelings and thoughts and instincts are channeled into a single path by forces that prove too great for him in the end. In becoming a Party clone, he becomes a chilling symbol of human manipulation taken to extremes.