Explain “mind of the Party” O’Brien’s argument about reality on page 249.

O'Brien's point is that the mind controls reality. The brain interprets the stimuli it encounters. If the party can control the mind, then the party can control reality. Reality becomes whatever the party says it is.

"We control matter because we control the mind. Reality is inside the skull. You will learn-by degrees, Winston. There is nothing that we could not do. Invisibility, levitation-anything. I could float off this floor like a soap bubble if I wished to. I do not wish to, because the Party does not wish it. You must get rid of those nineteenth century ideas about the laws of nature. We make the laws of nature."