4. _Renting wealth and borrowing capital have the same economic purpose, but the capital contract presents certain peculiar features._ In the interest contract for the loan of capital the interest always is and must be expressed in money; the capital sum must be expressed as value; and the interest rate expresses the relation between these two values. In each of these features the interest contract is in contrast with the renting contract. While the rent itself may or may not be expressed in terms of money, the value of the rented wealth is not so expressed, and there is no rent-rate expressing the relation between the two values. [Sidenote: The wealth concept and the capital concept contrasted] As here presented, the essence of the capital concept is in the mode or form of expression of wealth, not in the physical nature, the origin of its value, or any peculiarity in the kind of wealth; the content of the concept is limited only by man's thought of wealth, every good becoming capital when it is capitalized, that is, when the totality of its uses is expressed as a present sum of values. The difference between the wealth concept and the capital concept is therefore subjective, not objective; it is a difference in the mode of man's thought regarding wealth. The rent contract and the interest contract are modes of borrowing and lending which reflect this difference of conception. In their effort to express more exactly to themselves and to others the relative felt importance of their environment, men take in turn different points of view, and use different modes of expression. The most developed and exact of these devices for the social expression of valuations, which became possible only with a money economy and widened markets, is the capital concept, whose nature has been analyzed here. [Sidenote: The capital concept now prevalent] Summarizing the thought of this chapter, it may be said that the capital concept has gradually developed with industry, and is now the most widely prevailing mode of expressing the quantity of wealth. It is used in the discussion of all the most important problems of modern industry. The questions of income from wealth, of trusts and corporations, nearly all that is most notable in the development of modern industry, require the use of the capital concept. Yet (returning to the thought with which this chapter started) in many of the outlying districts other modes of looking upon wealth are employed. References to modern industry must be understood usually as applying to the most developed capitalistic conditions. CHAPTER 15 THE CAPITALIZATION OF ALL FORMS OF RENT § I. THE PURCHASE OF RENT-CHARGES AS AN EXAMPLE OF CAPITALIZATION [Sidenote: The nature and sale of rent-charges]