According to Ingsoc, the Party’s ideology, reality exists only in the mind of the individual. Do you believe an external reality exists independent of perception? If a tree falls in the forest and no one is around to hear it, does it make a sound?

I think that perception can be indoctrinated into people but even this has limits. Certainly societal pressure can make people believe any number of things. Violent and exploitive social modification is still used to day to 're-educate" or change people. At some point, the laws of nature must be observed. Despite what O'Brien says, he cannot levitate even if a person thinks they see this. Winston only loves Big Brother at the end of the book because his brains have been fried into scrambled eggs. Were he not so violently re-conditioned, would he still love Big Brother? The Party might be able to shut down all avenues of logic in the human brain but a tree still makes a noise when it falls in a forest weather humans are there to perceive it or not.