Debt has become a strange thing these last few decades. While the concept of debt is as old as social relations themselves, the new conception of debt isn't. And it is as new as it is strange.
It used to be a rather straightforward thing. Someone gave you a sum of money, and until you returned it (and a little extra for the hassle), you were in debt. For the longest time, this was the gist of it. A relation where one part owed the other some amount of money. With the implicit social understanding that this amount should be returned sooner rather than later.
A two-party system, if you will.
Then it expanded into a three-party system. As capitalism became a thing, and as regular people were expected to buy ever more expensive things, a system arose for the facilitation of purchases where the buyer couldn't actually afford the thing sold. Which might sound strange, until you remember that houses are both expensive and rather useless if no one actually lives in them.
Thus, home mortgaging arose.
Now, people had bought houses since the dawn of buying and houses. What happened (mostly in the US, but also elsewhere) was that the notion of owning your home became a propaganda hit. Everyone should own their place of residence, the proclamation went, and this sparked a great surge in market demand. The demand was, in fact, greater than the amount of people who could actually afford to buy.
Which, to be sure, is good if you want to sell the one singular house. But everyone wanted to buy one, and soon the supply of people who had enough money to buy one outright diminished. And it takes a long time to earn such amounts of money with honest work. Longer than anyone - buyers and sellers both - had the patience to wait. Especially the sellers - unsold homes are the opposite of profitable.
Thus, the three party debt system.
The one part is the buyer. The other is the seller. The third a bank. The bank lends the amount of money required to buy the house to the buyer, who then buys the house. The seller gets the money, and the buyer is indebted. Not to the seller, but to the bank.
The advantage of this is that it speeds up the buying/selling process. Profits happen faster for those who sell, and housing happens faster for those who buy. The latter will, of course, have to pay off this debt over time, but they will at least have somewhere to live during the process.
This does two things. First, it speeds up the rate of consumption. Buy now, pay later! - whatever the bought thing might be, and how long "later" might be.
Second, it transforms debt from a social to a legal relation. It's still debt, but it's also something stranger than it used to be. You still have to pay it, but you're not paying it to someone. You're paying it to something. Mostly a bank, but it might be hard to tell at times.
Being (re)paid a sum of money each month is a stable way to make a profit. Stable, but slow, and predictable. And being predictably slow is something you do not want to be these days. So banks got their institutional thinking caps on and started to ponder - how can we speed up the moneymaking process, and thus avoid being predictable, slow and boring?
At some point, the notion of selling the debts emerged. There might, after all, be someone else out there who wanted slow predictable, and if they could be persuaded to buy these debts from us, why not? We might not get as much money, but we'll get more money to use now, and more money now means faster profits - which is the same thing as more profits.
Thus, there are mortgages for sale. Buy now, payments later!
No longer do we see the three-party system we've grown used to, but rather a confusing polypoly party system where it can actually be quite tricky to find out just who you're actually paying your debt to. It might be the bank you once went to to get a loan, but it might also be someone or something else entirely.
Again, it's not a social relation. It's a legal relation. Which enables you, with a bit of bureaucratic legwork, to do something very strange: to buy your own debt.
There might be some implicit social understanding that frowns upon this maneuver. Something about the inherent value of paying one's debts. But, and this might sound strange: