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Among the constraints affecting economic trade-offs are those which depend on time. The choice between spending money on entertainment today and using that money to buy seeds to plant apple trees is not only a choice between two different sets of benefits; it is a choice among benefits to be received at two very different times. Other things being equal, the present is always preferred to the future, if only because life itself is uncertain and the future may never come, for the individual decision maker. Looked at another way, future benefits must be greater than present benefits to make it worthwhile to wait.³ There is some level of difference that will make present and future benefits equally valuable to a particular individual at a particular time. How much difference and how much time are matters that vary from person to person and vary incrementally with the same person. To someone dying of thirst a gallon of water right now might be more valuable than a swimming pool two years from now, even though the same person under normal conditions would prefer to wait for the pool. In short, with intertemporal substitutions, as with substitutions at a given time, there is no such thing as “the” rate of substitution, either in production or consumption. There is also no such thing as “the” value of a given object, for the time when that object is to be received changes its value. A swimming pool right now would be more valuable than a gallon of water even to a man dying of thirst. Clearly, then, it is more valuable than a swimming pool two years from now. More generally, any given asset is of greater value, the sooner it is to be received. The legal right to that asset can be sold for more in the market, the sooner it will become available. Apple trees that are half grown will sell for more than apple trees that were just planted, and apple trees that are fully grown will sell for the highest price of all, even if the only differences among these trees are the times when they were planted — which is to say, the time left before they produce apples.

Like so many important economic principles, the discount for time is so simple that it is readily forgotten in the rush of practical decisions or at the sound of heady rhetoric. For example, state and municipal governments in financial distress may unilaterally postpone payment on their bonds, with the assurance that those bonds will later be paid off “in full.” But even if this promise is carried out to the letter in money terms, the very fact that the bonds are paid off later means that they are not paid off “in full.” A hundred dollars three years from now is worth nearly twenty-six dollars less than a hundred dollars today, when the interest rate is 8 percent — and this does not even allow for inflation. In other words, a three-year postponement is economically the same as a confiscation of about one-fourth the value of the asset, even if there is no inflation. With even mild inflation, it can easily amount to a confiscation of, or default on, a third or more of the total amount entrusted to the government by those who bought its bonds.