However desirable a stable standard of value may be, its attainment is very difficult. Not only does no commodity exist the value of which is absolutely stable, but other necessary qualifications limit the choice. We may not select a commodity for a standard simply because its value is stable in a high degree. It must possess the other qualifications mentioned as well. Fortunately, however, these other qualifications contribute towards stability of value. Other things being equal, a commodity which is an object of universal desire, and consequently widely and generally used, is less liable to fluctuations in value than one which satisfies the wants of only a few people. The use of a commodity as a medium of exchange also is liable to steady its value, since it is possible to substitute other things for it if its value tends upwards, and to substitute it for other things if its value tends downwards. The quantity of the commodity thus used acts as a sort of reserve fund which is used from time to time to steady the market. It seems probable, therefore, that the commodity which has survived the various tests of actual experience, and is now the standard of value of the commercial world, is as stable in value as any single commodity which could be selected. That it comes far short of being an ideal standard, however, will be made evident in succeeding chapters.