Mostly for aficionados of the finer points of economics and its history. From John Stuart Mill’s Principles:
If we assume the quantity of goods on sale, and the number of times those goods are resold, to be fixed quantities, the value of money will depend upon its quantity, together with the average number of times that each piece changes hands in the process . The whole of the goods sold (counting each resale of the same goods as so much added to the goods) have been exchanged for the whole of the money, multiplied by the number of purchases made on the average by each piece. Consequently, the amount of goods and of transactions being the same, the value of money is inversely as its quantity multiplied by what is called the rapidity of circulation. And the quantity of money in circulation [M] is equal to the money value of all the goods sold [PQ], divided by the number which expresses the rapidity of circulation [V]. (Mill  1871: 494 – letters in parentheses inserted into the text)
As an equation:
M = PQ/V
Or as more commonly stated:
MV = PQ
Following that para are a number of considerations that help make additional sense of the point and also make it more comprehensible in relation to the operation of an economy.