A glance at any prominent current textbook will show how far economics still is from incorporating Fetter's insights. The textbook discussion typically begins with an exposition of the marginal productivity theory applied to wage determination. Then, as the author shifts to a discussion of capital, "interest"
suddenly replaces "factor price" on the y-axis of the graph, and the conclusion is swiftly reached that the marginal productivity theory explains the interest rate in the same way that it explains the wage rate. Yet the correct analog on the y-axis is not the interest rate but the rental price, or income, of capital goods. The interest rate only enters the picture when the market price of the capital good as a whole is formed out of its expected annual future incomes. As Fetter pointed out, interest is not, like rent or wages, an annual or monthly income, an income per unit time earned by a factor of production. Interest, on the contrary, is a rate, or ratio, between present and future, between future earnings and present price or payment.