Why is Winston beaten so much as part of his treatment?

Winston is viciously beaten for days to strip him of reason and the will to mount an intellectual argument. Winston notes that there is nothing worse than pain. In the face of intense pain, there are no heroes. O'Brien wants to begin "re-educating" Winston when he has nothing left to prove, say, or rationalize. O'Brien wants Winston beaten to a pulp so he can re-shape him again.