The law of diminishing marginal utility also tells us that if Crusoe is rich (that is, if he has many coconuts), then he will attach less value to a marginal coconut than if he were poor—provided that his context, including his stocks of other goods, remains otherwise identical. The proof of that law, it should be remembered, depends on the value attached by a particular individual to the various services a unit of a good can provide. Because we have no basis for equating or comparing the values of those services to two separate individuals, we cannot logically conclude that a coconut—or any other monetary unit—has lower subjective value to a rich person than to a different person who is poor. (The nature and characteristics of money will be addressed in detail in a later subsection.)

Interpersonal comparisons of value are not merely beyond human knowledge. The problem is more fundamental, for such comparisons are metaphysically meaningless: they do not correspond to any fact of reality. Crusoe's and Friday's value scales are incommensurable in the same sense as apples are incommensurable with oranges, and hours are incommensurable with gallons. (Cf. pp. 1.3:28-9.)