An Exchange Will Take Place When Two Commodity Units Are Placed in a Different Order on the Value-Scales of Two Different Persons

Acts of valuation are not susceptible of any kind of measurement. It is true that everybody is able to say whether a certain piece of bread seems more valuable to him than a certain piece of iron or less valuable than a certain piece of meat. And it is therefore true that everybody is in a position to draw up an immense list of comparative values;  a list which will hold good only for a given point of time, since it must assume a given combination of wants and commodities. If the individual’s circumstances change, then his scale of values changes also.

But subjective valuation, which is the pivot of all economic activity, only arranges commodities in order of their significance; it does not measure this significance. And economic activity has no other basis than the value-scales thus constructed by individuals. An exchange will take place when two commodity units are placed in a different order on the value-scales of two different persons. In a market, exchanges will continue until it is no longer possible for reciprocal surrender of commodities by any two individuals to result in their each acquiring commodities that stand higher on their value-scales than those surrendered. If an individual wishes to make an exchange on an economic basis, he has merely to consider the comparative significance in his own judgement of the quantities of commodities in question. Such an estimate of relative values in no way involves the idea of measurement. An estimate is a direct psychological judgement that is not dependent on any kind of intermediate or auxiliary process.

--Ludwig von Mises, The Theory of Money and Credit, trans. H. E. Batson (Indianapolis: Liberty Fund, 1981), 52-53.