Perhaps Bob Murphy could school her, since he wrote:
We need to explain why money has a certain exchange value on the market. It won't do (so these economists thought) to merely explain this by saying people have a marginal utility for money because of its purchasing power. After all, that's what we're trying to explain in the first place—why can people buy things with money?and
Mises eluded this apparent circularity by his regression theorem. In the first place, yes, people trade away real goods for units of money, because they have a higher marginal utility for the money units than for the other commodities given away. It's also true that the economist cannot stop there; he must explain why people have a marginal utility for money. (This is not the case for other goods. The economist explains the exchange value for a Picasso by saying that the buyer derives utility from the painting, and at that point the explanation stops.)
People value units of money because of their expected purchasing power; money will allow people to receive real goods and services in the future, and hence people are willing to give up real goods and services now in order to attain cash balances. Thus the expected future purchasing power of money explains its current purchasing power.
But haven't we just run into the same problem of an alleged circularity? Aren't we merely explaining the purchasing power of money by reference to the purchasing power of money?
No, Mises pointed out, because of the time element. People today expect money to have a certain purchasing power tomorrow, because of their memory of its purchasing power yesterday. We then push the problem back one step. People yesterday anticipated today's purchasing power, because they remembered that money could be exchanged for other goods and services two days ago. And so on.
So far, Mises's explanation still seems dubious; it appears to involve an infinite regress. But this is not the case, because of Menger's explanation of the origin of money. We can trace the purchasing power of money back through time, until we reach the point at which people first emerged from a state of barter. And at that point, the purchasing power of the money commodity can be explained in just the same way that the exchange value of any commodity is explained. People valued gold for its own sake before it became a money, and thus a satisfactory theory of the current market value of gold must trace back its development until the point when gold was not a medium of exchange.
The critical difference between fiat and commodity money is that fiat money can be produced in virtually unlimited quantities at very low cost. In this respect, the person who controls the printing press of a fiat currency is in a much stronger position than the person who owns a gold mine. With just some ink and paper, the printing press can create a million new dollars quite easily, whereas the owner of the gold mine would need to hire workers to operate expensive equipment in order to bring forth new amounts of gold having the same market value.