Since the values of things can never rise or fall simultaneously, every rise supposing a fall, and every fall a rise, it follows that the values of all taken together must be constant; in other words, that general values cannot change. Thus it is that we find whether any one thing has risen or fallen in value, as between one period and another, only by comparing it with all others,--in short, by its general exchange or purchasing power. If this has increased, then its value has risen; if it has decreased, its value has fallen. It is evidently not necessary that anything should exchange for more or less of _every_ other thing to show a rise or fall of value, but only that it should, on the average, exchange for more or less of all; that its average purchasing power should be greater or less. If it has exchanged at different times for the same amounts, on the average, of all other things, its value, clearly, has remained constant. This is the only standard, or test, which can be applied to the exchange value of any commodity to determine its constancy or variability, and it is inherent in the very definition of exchange value. The values of commodities may be compared to the surface of the ocean, which, vexed by winds and tides, is never at rest, every point continually rising or falling as compared with others. As some points rise others fall, yet there is a mean level which does not vary, and by comparison with which the variations of level of any particular point may be determined. So with values, there is a mean or average which is constant, and by referring individual values to that we can determine their fluctuations. These ideas will become clearer as we proceed to apply them concretely to the special case of money. Although there can be but one real _standard of value_, invariable at all times and places, yet, as before stated, any commodity may serve as a _measure of value_, and the great convenience subserved, by all the people of any locality or country using the same commodity instead of a number of different ones for this purpose, early led to the adoption of some one commodity in each locality as a "money" to measure values and facilitate exchanges. CHAPTER II. MONEY. _Definition of Money._ Money has been variously defined by different writers. Perhaps the definition given by Prof. F. A. Walker, though lengthy, is the most comprehensive. He says: "Money is that which passes freely from hand to hand throughout the community in final discharge of debts and full payment for commodities, being accepted equally without reference to the character or credit of the person who offers it, and without the intention of the person who receives it to consume it, or enjoy it, or to apply it to any other use than in turn to tender it to others in discharge of debts or full payment for commodities."