The regression theorem was created/discovered by Mises to explain the origin of money. It regresses from yesterday’s value of money back to the first time money was made to have a value. He said that money got its value from the last time barter that goods were traded for goods and the most valuable good became money.

The value of money is always and everywhere subjective. It depends on what you think is more valuable, the money you earned or the good or service you desire. Then, you will only buy when you think the money is worth less than the thing you want to buy. Then and only then will you buy. Never when you think the money and the good are the same value. It is all subjective and depending on you at the time you make the decision.