(Follow-up to part 1 and part 2.)
Let’s clear up some potentially confusing points about fiat money on our island:
We said that when the government spends, it increases net dollar wealth, and that when it taxes, it decreases net dollar wealth.
So you can think of the total government debt as the “national savings account”. But it is important to note that when we say "savings", we are really referring to savings in the form of dollars (or Treasuries, which we’ll get to later).
If I chop up some wood and build a bird house for my pet woodpecker, I've created wealth and my net worth (assuming that other people are interested in buying the bird house) has gone up. But my dollar wealth is unchanged. Since the total number of dollars out there is the same but there are more goods it can buy (because that spectacular bird house has been built), overall prices might have to come down. So you could argue that the government has to issue more dollars to keep pace with the amount of “real wealth” that is created.
Where does the extra money "come from"? Is the government racking up a bunch of debt?
You might be confused at this point as to where the money used for deficit spending comes from. There are two possible answers: the government can "print" the money, or it can "borrow" the money.
Let’s start with borrowing, since that’s the case most familiar to a lot of us.
In order to borrow money, the government has to print up a bunch of IOUs and sell them. We'll call the island IOU's "Treasuries" to match the U.S. terminology. So the government prints some Treasuries and exchanges them for cash with "regular" civilians. For now, those civilians represent island citizens, but later we'll consider what happens if they are foreigners.
For instance, if the government collected $500,000 in taxes but wants to spend $520,000, it could issue $20,000 worth of Treasuries and sell them to island residents. In the process, the government has $20,000 worth of cash, and it can proceed to spend or give away the money. If Lenny the Lender purchased $100 worth of government Treasuries, then he's just as wealthy now as he was before. His cash has been reduced by $100 but he now has $100 worth of Treasuries.
So now when we refer to "dollar savings" we'll be referring to cash or Treasuries. Both are a form of dollars; the difference is the Treasuries are scheduled to "turn into" regular cash at a future date. Alternatively, you could say that cash is just a "zero maturity" Treasury, i.e. a Treasury that gets paid back by the government right now.
So how does “printing” money work?
Well, on the island, we can literally print it and spend it. It is our imaginary island after all.
But does the U.S. actually just print money and spend it?
This is a minor point, but the U.S. Treasury works slightly differently. (Thanks to ESM for spelling this out for me.) If the U.S. Treasury wants to "print money", the Treasury issues bonds to the Federal Reserve, and the Fed pays cash to the Treasury. Now the Treasury has cash to spend, and the Fed is holding onto a bunch of bonds. The Fed can either hold the bonds, sell them on the open market, or tear them up. In the last case (which is quite common), we can say that the Treasury effectively printed money, and for some reason, the amount that it printed is not added to the U.S. debt. This is horribly inconsistent, but it’s the way it is. It's as the government periodically get loans and the bank keeps forgetting to collect debt payments.