But price is quite another thing. The price of a commodity is the ratio at which it will exchange, not for all other goods and services but for one specific thing, namely, money. Price is value expressed in terms of the medium of exchange. We habitually translate our economic goods into terms of money before we buy or sell them. A general rise or fall in prices is quite possible, for this is merely another way of saying that money will buy less or more of all other things. It is immaterial whether we say that prices have gone up or that money has gone down; we mean exactly the same thing. [Sidenote: What determines the level of prices?] =Competition and Monopoly.=—Exchange is conducted, for the most part, under free competition. Buyers give as little as they can in money for goods; sellers get as much as they can. When goods bring higher prices, more will be produced until prices are forced down again; if prices fall, production will decline until the reduction in supply serves to bring them up again. This is the theory of free competition. In practice, however, it does not always work so automatically. Some things, such as diamonds and platinum, cannot be produced in unlimited quantities no matter how much labor, capital, and organization we may apply. Other things are _legal_ monopolies, or patented articles, which can be produced by only one concern and are not subject to the direct influence of competition. Still others are _natural_ monopolies due to the fact that from the nature of things only one concern can produce the goods or render the service. A telephone company, for example, has a natural monopoly. Competition involves a complete duplication of the service. It means that many subscribers have to put two telephones in their stores or homes in order to get into full touch with other users of telephones. The net cost of telephone service to customers cannot be reduced in this way. Finally, some things are the subject of artificial monopoly, that is to say, they are produced or distributed under arrangements which restrict or eliminate competition (see pp. 386-388). [Sidenote: The effects of monopoly.]