Fisher saw that subjective economic value is not only a function of the amount of goods and services owned or exchanged, but also of the moment in time when they are purchased with money. A good available now has a different value than the same good available at a later date; value has a time as well as a quantity dimension. The relative price of goods available at a future date, in terms of goods sacrificed now, is measured by the interest rate. Fisher made free use of the standard diagrams used to teach undergraduate economics, but labeled the axes "consumption now" and "consumption next period" (instead of the usual schematic alternatives of "apples" and "oranges"). The resulting theory, one of considerable power and insight, was presented in detail in "The Theory of Interest".