Value & Capital, CHAPTER XIII, Section 5

In this section, the final one of the chapter, the author summarizes the chapter as having given a "preliminary survey of the relation between money and interest."  The author notes that "The fact that money and securities are close substitutes is absolutely fundamental to dynamic economics."  And he goes on to note that "This close substitutability is much the most important property of actual money which we shall need in our further inquiries."

Money serves as a standard of value, although from the point of view of economic analysis, it is straightforward to consider some other standard commodity "selected from the rest to serve as standard of value."  The author notes that actual money also has two other important properties, "considered for the first time in this chapter," namely "the familiar properties of being a 'medium of exchange' and a 'store of value.'"  He concludes the section, and the chapter, by stating that the "important consequence" of these properties of money "for the working of the price system is simply this:  they explain why there is such a close relation of substitution between money and securities, that is to say, they explain the phenomenon of interest -- money interest."