I always thought the Quantity Theory of Money was a discovery of the 18th century Enlightenment, one of the first intellectual achievements of the new science of political economy.
However, I recently stumbled across a “Treatise on Money“ by the 16th century Jesuit theologian Luis de Molina which contains, among other economic ideas, a concise statement of the quantity theory as well as some empirical evidence for it.
Molina is best known for coming up with a clever solution to the theological problem of reconciling the omniscience of God with the free will of humans: God, Molina reasoned, knows exactly how humans would behave in any given hypothetical situation (this kind of knowledge Molina called scientia media, „middle knowledge“). In other words, God is the perfect economist: He has complete knowledge of all His creatures’ preferences, their beliefs and their cognitive biases, and therefore can predict what choices they will make freely when faced with any possible budget constraint. This idea helps solving a number of important theological problems, like the issue of predestination or the theodicy.
Anyway, Molina was not only a great theologian, but also a superb economist. For instance, he clearly understood the logic of supply and demand in determining market prices and also saw the logic of no-arbitrage conditions. And here is his explanation of differing price levels in different places:
There is another way that money may have more value in one place than in another: namely, when it is more abundant. In equal circumstances, the more abundant money is in one place so much less is its value to buy things with, or to acquire things that are not money. Just as the abundance of merchandise reduces their price when the amount of money and quantity of merchants remains invariable, so too the abundance of money makes prices rise when the amount of merchandise and number of merchants remain invariable, to the point where the same money loses purchasing power.
And here is his evidence for the theory:
So we see that, in the present day, money is worth in the Spanish territories much less than what it was worth eighty years ago, due to the abundance of it. What was bought before for two today is bought for five, or for six, or maybe for more. In the same proportion has the price of salaries risen, as well as dowries and the value of real estate, revenues, benefices, and all other things. That is exactly why we see that money is worth much less in the New World, especially in Peru, than in the Spanish territories, due to the abundance there is of it. And wherever money is less abundant than in the Spanish territories, it is worth more. Neither is it worth the same in all parts because of this reason, yet it varies according to its abundance and all other circumstances. And this value does not remain unaltered as if it were indivisible, yet fluctuates within the limits defined by the people’s estimation, the same as happens with merchandise not appraised by law. This money’s value is not the same in all parts of the Spanish territories, but different, as ordinarily it is worth less in Seville—where the ships from the New World arrive, and where for that reason there is usually abundance of it—than what it is worth in other places of the same Spanish territories.