2. _Capital to-day may be defined as economic wealth expressed in terms of the general unit of value._ In economic discussion new conditions must be recognized and an attempt made to adapt definitions to the language and needs of practical life. By this definition, capital, at any given moment of time, includes all economic goods in existence, when they are thought of in terms of their value. But things have different durations, some are parts of the capital of the world only for an instant, others for a week, a month, or years. Most capital is composed of things durable in a large degree. It has been seen above that there is no reason for keeping things unless they will increase in value, that is, unless a rental is logically attributable to them. Everything kept for a day, a month, a year, is kept because thus it will continually give off uses or by accumulating them it will become more useful. Hence, when interest is defined as the payment for the use of capital, it is connected with all wealth that is expressed in the capital form. In practical business and in theoretical discussion this is the idea of capital that alone can be consistently followed. Capital is the value equivalent of a sum of money "invested," "clothed" in forms of wealth purchased and exchanged. Wealth has become fluid in modern times; it was crystallized in medieval times. Under the new conditions, wealth, expressed in the mobile form of capital, flows into the most distant corners of the industrial world. [Sidenote: Distinction between money and capital] 3. _Capital must not be identified with money although it is expressed in terms of money._ While money and capital are not identical, neither are they opposite or mutually contradictory. Money is but one species of the genus capital. It is a particularly durable form when industry as a whole is considered, a particularly fleeting form in the individual's possession, and a particularly important, though not necessarily the most important, form in its social significance. The things composing capital are concrete things, scarce forms of wealth, some of which are yielding gratification at the present moment, or are destined to do so at some future moment; others of which are not themselves giving direct gratification, but are indirect agents for the gratifying of wants. To this latter group belongs money. The caution contained in this proposition may appear to some to be superfluous, but it is most needed. The mind is so prone to identify things that are expressed currently by the same words. The ease with which money and capital are thus confused has led to various popular fallacies on practical economic questions. [Sidenote: Contractual interest and rent involve a difference of business procedure]