These various factors change slowly, and the quantity theory answers the question: What general change occurs in prices as a result of the increase or decrease of the money in a given community at a given moment?
Like the law of gravitation and the law of projectiles, the theory must be interpreted with relation to actual conditions.
The quantity theory makes intelligible the great and rapid changes in prices which have followed sudden changes in the quantity of money.
Inductive demonstration of broadly stated economic principles is usually difficult, but there have been many "monetary experiments" to teach their lessons.

Many inflations and contractions of the circulating medium have occurred, now in a single country, again in the whole world; and the local or general results have helped to exemplify richly the working of the quantity principle.

With the scanty yield of silver and gold mines during the Middle Ages, prices were low.

After the discovery of America, especially in the sixteenth century, quantities of silver flowed into Europe.

The great rise of prices that occurred was explained by the keenest thinkers of that day along the essential lines of the quantity theory, tho there were many monetary fallacies current at that time.