There is also in agents an intensive margin, beyond which are certain unexploited uses in the things that we already have. This is a more subtle thought, but it has been already discussed in connection with diminishing returns. These potential uses in agents, uses which in the existing conditions lie outside the margin of utilization, of course have no value. We have noted that there is an equilibrium between these two margins. Rent is measured from a zero point of utility either in a good, or in other poorer grades of goods. A corollary of this proposition is that there is a limit to the rental that anything can yield under any given condition. Below the present margin of utility of any goods there exist great quantities of free goods, unused goods, or unexploited uses. It is only uses above this margin that yield rent. Rent is the difference between the value of the better grades and the value of the free goods. It is therefore due to the limitation in the supply of indirect agents of the better quality, or to the scarcity of the more effective uses in those agents. [Sidenote: Restatement of rent, economic and contract] [Sidenote: Economic rent is primary] 5. _Rent may be redefined as the value of the scarce uses of wealth within a given period._ Rent is the felt importance of the usufructs of agents in securing gratification. It is measured by the marginal utility of any particular grade of agents in securing products. These definitions and the discussion throughout this chapter applies to economic rather than to contract rent. In fixing and agreeing on contract rent, men are seeking to estimate the importance of indirect goods, the importance that an agent will have in getting a product. They are bidding for the use of things, and what they bid is contract rent. Contract rent is based on the existence of economic rent. Economic rent does not depend on contract rent, but on the differences in the effectiveness of agents to secure a given product. If there were not differences in the product, and no limits to the supply of indirect agents, rent could not exist; it would be inconceivable. But these differences existing, economic rent inevitably arises, for men cannot keep from attaching value to the things that affect their desires. Contract rent in turn appears wherever the use of wealth becomes an object of exchange and agreement between men in a free society. CHAPTER 11 REPAIR, DEPRECIATION, AND DESTRUCTION OF WEALTH: RELATION TO ITS SALE AND RENT § I. REPAIR OF RENT-BEARING AGENTS [Sidenote: The necessity of repairing nearly all economic agents]