O'Brien informs him of the why behind the mind control of The Party. The goal is pure power itself. The individual is mortal and can never have power alone, but when he destroys his own identity and relinquishes control to the Party, he will live and be powerful forever. The Party is omnipotent. We control matter because we control the mind.
Reality is inside the skull.
Part 3, Chapter 3, pg. Reality Control 12 Power, O'Brien says, is defined by the ability to make other human beings suffer. In order to maintain power, The Party must remove all pleasures of the individual. The vision of the future will be a boot stamping on a human face, forever. Heretics like Winston, rise only to be defeated, humiliated, and realigned by The Party.