[AnimalMuppet]

Let's say we have X amount of dollars. Time passes. The population increases, productivity increases, and now the economy (measured in actual stuff) is twice as big as it was. How many dollars should we now have?

If we have X, then each dollar is worth twice as much stuff (or, put differently, everything costs half as much). That's not a great outcome if, for example, you borrowed money to buy a house, and that loan specifies repayment of a fixed number of dollars. On the other hand, if you hid some dollars under your mattress, it's great. But it's unclear why society should want hiding dollars under your mattress to be the ideal investment strategy. That's not going to be optimal for society as a whole.

Or, we could have 2X dollars. Then each dollar will buy the same amount of stuff as before. That seems more reasonable. To do that, though, we have to increase the number of dollars in proportion to the growth of the economy as a whole.

(It's not that simple, of course. The velocity of money also matters. And the Fed is trying for 2% inflation, not zero.)