But two or more quite different things may be expressed in terms of another thing and so be made comparable. Money becomes the value-unit through which different things may be reduced to the same terms for comparison. With this mode of expressing the value-equivalence of various goods, the interest contract first becomes possible, money (the standard of deferred payments) being the thing exchanged (possibly only in name) at two periods of time. What is really compared are various gratifications which may be produced by very different material things or services. In its last analysis comparison of values at different periods of time must be a comparison of psychic incomes, of two sums of gratification. The comparison of the value of a bushel of apples with that of a barrel of potatoes or a suit of clothes at the same moment appears simple enough. When all are expressed in terms of money, the comparison of each with its value-equivalent at a later date becomes easy. The simplicity and obviousness of time-value in the case of money loans at interest led men at first to recognize that phase of the problem exclusively, and later the term "interest," not without much confusion of thought, was given a wider significance. Let us now see how large a part of the whole problem of time-value is outside of the money loan. [Sidenote: Time-value is involved in capitalization of land] 2. _The problem of time-value is quite separable from the concepts of money and capital, though usually connected with them in practice and theory._ It is true that the problem of time-value was first clearly recognized in connection with money and a formally expressed capital sum. Misled by this fact, and taking a very narrow view, writers seventy-five years ago recognized but dimly the problem of time-value in connection with the valuation of the incomes derived from land. It is true, as has been shown above, that the mere putting of an estimate on a durable good such as land involves the process of capitalization, which in turn implies a comparison of the values of the rents expected at different periods. Diminishing returns in the use of agents involves a loss of time to secure the usufructs emerging. The relation of these facts was not clearly seen until of late. The phenomenon of time-value as above defined may be seen to be broader even than that of capitalization. The difference in the value of the successive rents of wealth must have been recognized and in some degree measured before there was any conscious calculation of capital value. Differences in value due to time are everywhere. The problem of time-value often is present where money is not even spoken of or thought of. Money no more causes this time-difference in value than balances cause weight. [Sidenote: Time-value is taken account of in the keeping up of repairs]