Without private ownership of capital, private capital will never be invested in productive enterprises. With me so far? If we decree that a worker might get wages, and save his money, but cannot spend that money to increase his productivity it would be pretty silly right?
A carpenter takes some of the money he gets from building a deck and buys a new power saw with it. Why would he do such a thing? Because with his new power saw he can work faster and produce the same output for less effort. He takes his current excess money and spends it on capital, in the hope that the money he spent now will bring him even more money in the future.
I hope we can agree that this sort of personal investment is only sensible and logical, correct?
Now imagine that the carpenter doesn’t have any extra money to buy a power saw. But his buddy does. And his buddy offers to buy him the saw, because with the saw the carpenter can make more money and pay back the buddy in the future. One possibility is that next year once he’s made more money, the carpenter should pay his buddy back exactly what the buddy paid for the saw. His buddy paid 50 shekels for the saw, so he should be paid back 50 shekels. And this was the custom in many pre-capitalist societies, even going so far as to declare that anything else was displeasing to the gods and make the spirits angry.
But why would your buddy lend you 50 shekels for a year, if he only gets back 50 shekels? Because he’s your buddy, or your family, or someone you have a close social relation with. He loans you the money, not because he makes money from lending money, but as a favor. In fact he’s losing money–he could buy the saw for himself and use it to make even more money for himself, but instead he gives it to the carpenter.
So we see the problem. No one will lend money except as a favor, which means there is no way to borrow money except from people within your close social group. And the inability to borrow money is a very high barrier to economic development. Finance isn’t just an afterthought, it is technology that was a necessary precondition to the industrial revolution.
Without private capital–the ability to loan money to existing enterprises, and the ability to buy and sell existing enterprises–our modern economy just wouldn’t work. Real world attempts to do away with private capital have turned out to be pretty much disastrous.
You are absolutely correct that money and ownership are legal and social fictions. They exist only in the minds of human beings, because we agree to pretend as if they exist. And we very often stop pretending these things exist, and we get barbarian hordes looting and pillaging. But a barbarian horseman who robs a city and enslaves the citizens and refuses to pretend that the citizens have ownership over themselves or their farms and houses and so on, will at the same time absolutely respect his fellow barbarian’s claim of ownership over particular horses, weapons, slaves, treasure, and so on.
Because unless the barbarians work together as a group there’s no way they can pillage a city, and one way they work together is to agree that Thag’s horse belongs to Thag, and Tork’s scimitar belongs to Tork, and Vong’s slave belongs to Vong. Tork’s scimitar is a capital good that enables him to kill city-dwellers and steal their goods. Without his sword, Tork cannot rob as efficiently. And if Tork didn’t have a sword, it would be in Thag and Vong’s interest to somehow provide him with one, so he can help them pillage.
And so we have the comical spectacle of the slavemaster insisting on the sacred rights of property that allow him to own the bodies and labor of other human beings, who do not own themselves, and the other slavemasters agreeing.
But of course, ownership and property do not descend from God, or from nature, but are just ways that human beings organize their activity. The reason we agree to pretend that Bob owns a store even though he doesn’t work in the store is that ownership of this kind is a consequence of allowing private capital, the investment of previously accumulated money into productive enterprises. We need capital, therefore we need some source of capital. We can restrict people to providing their own capital, except through the rare public provision of capital goods and improvements. But for industrial civilization it turns out we need more than that, we need private capital. When we try to do away with it, as people in the 19th and 20th century sometimes argued, we find that it doesn’t work very well.
Not that private capital is any sort of panacea. You need all sorts of social institutions as well, otherwise you have the rich stealing from the government and the government stealing from the people, and on and on.