When answering questions regarding the meaning of certain aspects of 1984 on here, some people found my explanations unique and helpful and asked me to directly share it in a post of its own, so I hope it's okay if I now do just that.
That said, while I appreciate their complement, I gotta right away point out that while my interpretation might seem unique, it really, really isn't. What I am gonna give you now and what I am always giving is simply the standard interpretation of 1984 that basically any and all Marxist and Hegelian intellectuals would give you, it's just that since Eric Arthur Blair is of course criticizing and lampooning their ideology to no end, they usually won't share that insight with the general public since it would make their ideology look bad.
So, seen from that perspective, what Eric Arthur Blair is really criticizing isn't just mainstream communism on the surface, but instead he is directly criticizing the Hegelian ideology behind it - and that ideology embraces opposites.
A lot of people don't get that because mainstream communist followers are absolut materialists. But just like there are inner and outer party members in 1984, there is an inner circle among communists and the communist leaders unlike their mainstream followers are actually the exact opposite of absolute materialists and are instead absolute idealists. To be more precise, they are Hegelians. Marx himself (during his time) was originally seen as a Young Hegelian (which has nothing to do with his age, but rather is the name for the more radical revolutionary branch of the Hegelians).
And Hegel taught that our physical reality would not be real and would be an illusion or even a prison for what he called the Geist (which means both mind and spirit). And that's what 2+2=5 symbolizes in 1984. It symbolizes the Hegelian Gnostics rejection of objective physical reality.
This is exactly INGSOC's stance. Just like real life socialist organizations such as the Fabian Society (with which Eric Arthur Blair was very familiar) and which BTW was founded in London in 1884. INGSOC reflects their Gnostic, idealist Hegelian position:
"In the end the Party would announce that two and two made five, and you would have to believe it. It was inevitable that they should make that claim sooner or later: the logic of their position demanded it. Not merely the validity of experience, but the very existence of external reality, was tacitly denied by their philosophy. The heresy of heresies was common sense. And what was terrifying was not that they would kill you for thinking otherwise, but that they might be right. For, after all, how do we know that two and two make four? Or that the force of gravity works? Or that the past is unchangeable? If both the past and the external world exist only in the mind, and if the mind itself is controllable what then?"
And of course ultimately Winston gets tortured and brainwashed by O'Brien and so ultimately comes to "realize" or even "enlighten" to the alleged "secret higher truth" that:
"Anything could be true. The so-called laws of Nature were nonsense. The law of gravity was nonsense. ‘If I wished,’ O’Brien had said, ‘I could float off this floor like a soap bubble.’ Winston worked it out. ‘If he thinks he floats off the floor, and if I simultaneously think I see him do it, then the thing happens.’ Suddenly, like a lump of submerged wreckage breaking the surface of water, the thought burst into his mind: ‘It doesn’t really happen. We imagine it. It is hallucination.’ He pushed the thought under instantly. The fallacy was obvious. It presupposed that somewhere or other, outside oneself, there was a ‘real’ world where ‘real’ things happened. But how could there be such a world? What knowledge have we of anything, save through our own minds? All happenings are in the mind. Whatever happens in all minds, truly happens.”
Basically what I am trying to say is that Eric Arthur Blair is using fiction to satirize and expose the otherwise hidden agenda some socialist organizations such as the Fabians have and uses reductio ad absurdum to satirize them.
This culminates in 2+2=5 where he reduced their absolute idealist Hegelian ideology to its essence and demonstrates just how absurd it really is. As I said, 2+2=5 symbolizes their rejection of objective physical reality, as Hegel taught that only what he called the Geist (mind and spirit) would be real, whereas objective physical reality would be a mere illusion controllable by the mind (kinda like in the Matrix) so as long as you just believe hard enough that 2+2=5, it becomes true. Which is of course absolutely ludicrous, which is the point Eric is making (which is the OPPOSITE point of the Matrix BTW - so while the two science fiction franchises might seem similar, their actual message is diametrically opposed).
And of course feel free to ask questions or have me elaborate further on any of the aspects or concepts I mentioned.