Aside from that general acceptability, which is the very essence of money,--without which no commodity could be considered money, and which, therefore, all money may be considered as having,--the great requirements of money are _invariable value_, added to _convenience of form_, _size_, _weight_, _and value_. This latter requirement pertains to the function of a medium of exchange, and the degree in which it is possessed by the different money materials or kinds of money, depends wholly on the values to be transferred by its use. For small amounts, silver is preferable to either gold or paper; as the amount increases, gold becomes preferable to silver; and for all amounts above fractional currency, paper money is unquestionably more convenient in every way than either gold or silver, and the advantage increases with the amount. _Invariable value_ is the great requirement for both the functions,--"a measure of value" and "a standard of deferred payments." Indeed these two functions may practically be considered one; the only difference between them being centred in the element of time, and that is more or less involved in every exchange requiring the use of money, since some interval must elapse between the sale of one commodity and the purchase of another with the money received,--which constitutes the whole exchange transaction,--and during such interval the money should maintain a constant value. When the interval over which the transaction is spread is a large one, as in the case of notes and bonds, any variability is more noticeable than when the change is distributed among many holders of money. Before considering further the great necessity for invariable money value, it will be best to consider the laws and forces which determine and control the value of money. _Money Value._ That money is a commodity, and that its value varies like that of every commodity in accordance with the law of supply and demand, are incontestable. The fluctuations in the value of money can be detected, it is clear, in the same way that changes in the value of any commodity can be detected, by comparison with all other commodities,--by its average purchasing power, in short. The value of a commodity, when measured by money and expressed in terms of the unit of money, is called its _price_. If the prices of all commodities, or the average of all, rise or fall, it is conclusive evidence that the value of money has changed, for its purchasing power is less in the one case and greater in the other. Indeed the statement that general prices have fallen is equivalent to saying that the value of money has increased, and _vice versa_. Therefore, if the value of money remains stable, average prices must remain constant. The following quotations will show that these views are correct, and that they are generally accepted by authorities on finance and political economy, though very commonly overlooked and neglected in discussions on the subject.