That is, prices rise and fall in direct proportion to changes in the total quantity.

This is a simple explanation of a complex and difficult set of conditions.

The phrase, "other things being equal," betokens the statement of a tendency where there are several factors.

The quantity theory explains what happens when there is a change in one of the factors--the number of pieces of money.

There are three large sets of facts to be brought into relationship with each other in the quantity theory: (1) the amount of business, or the number of trades effected; (2) the rapidity of circulation, depending on the methods by which business is done; (3) the amount of money available.