The other conception, called "objective value," or "exchange value," is dual in its nature, involving in all cases two or more commodities. Abstractly, it is _the ratio at which commodities may be exchanged for each other_, or, since such ratio for a unit of one commodity is expressed by the amount of another given for it, the exchange value of a thing is the quantity of some other thing that will be evenly exchanged for it, or, considered in a general sense, the amount of commodities in general it will exchange for,--_its general purchasing power_, in short. This latter conception--exchange value--is the one that principally concerns us in discussing the subject of money. It is also the conception generally in mind when the simple term _value_ is used either by economists or by the general public, and wherever the term is used in this work without qualification it is to be understood in that sense. The Austrian economist, E. von Böhm-Bawerk, says, in his "Positive Theory of Capital," p. 130:-- "Value in the subjective sense is the importance which a good, or a complex of goods, possesses with regard to the well-being of a subject." "Besides the expression 'value in exchange,' English economists use, quite indifferently, the expression 'purchasing power,' and we Germans are beginning in the same way to put in general use the term _Tauschkraft_." The value of a thing may be considered either in a particular sense, with reference to some other specified thing, or it may be considered in a general sense, with reference to all other things considered as a whole. We may say the value of a bushel of wheat is two bushels of corn, meaning that these two commodities exchange for each other in that ratio; or we may speak of the value of wheat having risen or fallen, meaning that its general purchasing power, or the ratio between that and all other things taken as a unit or a whole, has increased or decreased. The term must invariably be used or considered in a general sense, unless otherwise specifically stated, for we must always have some other thing in mind besides the one whose value we are considering; while if no other is stated, commodities in general (taken as a whole) is that thing. Value being a ratio, it is impossible for all values to rise or fall simultaneously. The sum of subjective values may increase or decrease,--indeed it is one of the great objects of human endeavour to increase the sum of want-satisfying power,--but the sum of the ratios between these subjective values is constant. As one term of any ratio rises relative to the other, the second necessarily falls as regards the first. This principle is so universally recognized that quotations might be given from almost every work on political economy in support of it. The following will be sufficient, however, as regards both the definition of value and this principle. John Stuart Mill says, in his "Principles of Political Economy":--