You are the only person who will get the use of your brain. No matter how magnificent it is, when you die, it will be used up. You will not be able to sell it to anyone at your time of death, and even if you could you would need your brain in order to be there to receive the money. It is not worth anything to anyone else even if you tried to commodify it. Your brain is the one thing you just can’t sell to anyone. The idiosyncratic marvels of your own particular brain have a necessarily limited interest to other people precisely because they by definition can’t ever experience them as the be-er inside that particular brain.
What is someone’s “rightful share” of access to the world’s collected means of production? An equal one. No one wants to be anyone else’s slave or subordinate either in name or practice. Everyone has the right to not be anyone else’s slave. This is something that every human can understand that every other human might reasonably want. If we are to have a peaceful society, we must accept a foundation for it on enduringly and truly equal access to political and economic power. Any other arrangement justifies war.