It's characteristic of economics that it makes a song and dance about summing a geometric series while completely missing the point about how the system actually works.
- banks take in cash in from depositors (and other creditors)
- banks lend out cash to borrowers
- banks do not lend out all the cash that they've borrowed - they keep some in reserve
Do you have the name of any retail bank which does not operate according to this model? What does their balance sheet look like? The "textbooks" reference is not relevant here - my experience is from working in retail banking operations.
https://www.sciencedirect.com/science/article/pii/S105752191...
Essentially the bank can make the loan with a zero balance sheet.
If you work at the back end of a retail bank operation then you'll know that the loan department and the treasury department do not talk to each other about quantities. They talk to each other about price. Other than that the two operations operate in parallel. 'Funding' is just swapping the liabilities around in the market to try and reduce the amount the bank pays out in interest.
https://www.cooperative-individualism.org/dodson-edward_revi...
Banks don't 'lend out cash', they create deposits. Deposits are liabilities for banks. When a bank takes on a deposit, it also takes on a corresponding asset: either a loan in its books, or an increase in its reserve account.
In practice the reserve ratio is unimportant for a bank: in Canada and the UK the ratio is 0%. The real constraint on a banks ability to create money is it's leverage ratio, i.e. (assets - liabilities)/assets. In reality the formula is more complicated and uses risk weighting by asset class, but that's the crux of it.
I’m not confusing anything. Believe it or not, banks actually have to follow double entry accounting rules, and follow regulations about capital adequacy. Most countries don’t even have a reserve requirement, deposits generally don’t factor into lending at all.
The only way it can work is if writing a mortgage creates an asset (the loan), and then a new liability is made to match it (a deposit of the same amount).
The security is 100% the bank’s capital, since any assets (cash/reserves) from deposits have matching liabilities.
The article I linked above from the UK’s central bank is confirming that all modem banks work the way I am saying.