This section occurs as Winston is being conditioned by torture to agree with the Party doctrine and worldview by O'Brien. Slowly and methodically, O'Brien takes Winston's beliefs one by one and attacks them until they are destroyed and Winston only accepts what he wants him to accept. This question relates to Winston's implicit belief that the Party will be destroyed at some stage. When O'Brien questions Winston further, to encourage him to name the "they" that will bring about the Party's downfall, Winston is only able to give the following proof of the surety of the eventual demise of the Party:
I believe it. I know that you will fail. There is something in the universe--I don't know, some spirit, some principle--that you will never overcome.
He goes on to define this "spirit" as "the spirit of Man," and it is at this point that O'Brien destroys this belief by forcing Winston to take a look at himself naked in a mirror, as the sole carrier of this "spirit of Man." The truly crushing element of this story is the way that Orwell uses O'Brien as the spokesperson for the party to extract any glimmer of hope from the situation and create a bleak, unyielding hopeless vision of the future that stretches on into eternity with the Party ruling supreme. Against such power, Winston's talk of a "spirit of Man" seems incredibly naive.