Winston writes in his diary that if there is hope in overthrowing Party rule, then it lies in the proles, the disregarded masses comprising 85% of the population of Oceania. They need to become conscious of their own strength.
The proles are largely untouched by the Party; they're not smart enough for the Party to bother brainwashing. Thus these prole people don't get what's going on enough to revolt.
Winston looks through a children's history book and copies the passage about capitalists into his diary. The Party claims in said passage that it has increased the standard of living from past times. But Sherlock Winston suspects this is a lie. Ultimately, there's just no way to tell.
Winston thinks (quite eloquently): when the past is erased, and the erasure is forgotten, the lie becomes truth.
Winston now recalls an occasion when he had proof that the Party was changing history. At one time, in 1973, Winston had held in his hands evidentiary proof that certain people who the Party deemed never existed had actually existed.
Unfortunately, Winston destroyed the proof.
Winston realizes the futility of physical evidence, and wonders whether the mind itself is controllable. He refuses to believe that it is, though, and realizes that the physical world exists so long as there is a mind to perceive it. Thus, he writes, freedom is the freedom to think.
Then he thinks, I should give this diary to O'Brien, that guy I have an intellectual crush on.