Aren't these two paragraphs contradictory?
If there's not enough cash in circulation, doesn't that mean that much of the money in people's bank accounts is not really money that they "have", but money that they borrowed? So that when they spend it, they are in fact spending money that they don't have?
Not at all. Most of the money that circulates isn't in the form of cash.
I get paid via direct deposit, and spend it via my credit/debit card. At the end of the month, the card company takes the money out of my bank account via direct debit. If it weren't for the quarters I need to buy coffee in the department office, I could go weeks without touching cash.
For that matter, I control a substantial amount of federal research funding, again all through a P-card. I've spent something like a half million of the government's dollars over the past several years, and I've never seen a single picture of a dead president. It's all done via P-card and/or purchase order.
... but only because the electronic/credit system exists to support that. I don't need to take
cash on a trip, because I can take my P-card and charge my business expenses to the National Science Foundation. From my perspective, this is actually much better. It's safer (if I'm robbed, I just cancel the card -- I'm not out the money), and I don't run the risk of running out of money if I have to change flights or something.
That actually happened to me in Japan a number of years back. JAL messed up my reservation, and I had to pull two grand out of thin air to get back to the States. I got it all back eventually, when we got the whole mess sorted out. Fortunately, I had a card on it that could take a sudden $2000 sucker punch without flinching.
Turn out your pockets. If, for whatever reason, you suddenly needed to spend $2000 to get home tonight, could you do it? I could. In fact, I suspect most of us could -- if we have a credit card with us. If I needed cash, I'd be hosed; I'm not sure I could come up with $10 for a cab ride.