4. _The essential thought in rent, as we shall use it, is that it is the value of the usufruct as distinguished from the value of the use-bearer or thing itself._ The meaning of usufruct is the use of the fruits, or in legal phrase: "the right of using and enjoying the income of an estate or other thing belonging to another, without impairing the substance." The obvious fact is that fruits can be eaten without destroying the tree, the harvest gathered without destroying the field. By a metaphor the word in legal discussion is applied to the use of any product, and we shall employ it, as in common speech, in reference to one's own goods as well as to the goods of another. [Sidenote: Rented agents are looked upon as durable] The qualities whose use gives value are not usually indestructible, but they are treated as undestroyed. There is a famous phrase used by Ricardo, "rent is paid for the original and indestructible qualities of the soil." He said "indestructible," but the word is not apt. There are many qualities in the fertile field that _must_ be destroyed when it is used. Every economist since Ricardo's time has recognized this, and many excuses for the inaccuracy have been given. After every harvest, the field is less serviceable than before, and if it is to be of the same grade of efficiency, the fertile elements must be restored. We cannot assert that Ricardo meant _undestroyed_, for he was not quite clear on the question. But it is evident that one can count as true income only that part of the value of product that remains after full repairs have been made. It is only by a fiction that most indirect agents can be regarded as indestructible. Things yielding rent are not indestructible, but generally they are preserved undestroyed. [Sidenote: True rent a net income] 5. _A distinction must be made between gross and net, or true and false rent._ Before the usufruct is estimated, allowance must be made for repairs, depreciation, and for various expenses which absorb a good portion of the gross product. When this allowance has been made, the income may be considered as a net sum not due to the sale, or to the using up of any part of the thing rented. This is the essential thought in typical rent--that it is the value of the surplus, or net product, of an economic agent leaving the agent itself unimpaired in efficiency. The total product is sometimes called the "gross rent," but economic rent is "net rent." This thought is made clearer by the following discussion. § II. THE HISTORY OF CONTRACT RENT AND CHANGES IN IT [Sidenote: Economic and contract rent distinguished]