It might be wondered whether money could be used to establish interpersonal comparisons of value. For instance, Crusoe, Friday, and a tribe of natives might all engage in trade, using coconuts as a primitive kind of money. If Crusoe is willing to pay ten coconuts for a tree house but Friday is not, could we perhaps conclude that the tree house has greater subjective utility to Crusoe than to Friday? The fallacy in this argument is that we cannot compare the utility of a coconut to Crusoe with its utility to Friday. In fact, we know from the law of diminishing marginal utility (pp. 4.4:29-31) that the utility of coconuts varies even within Crusoe's value scale, depending on his present stock of coconuts and other conditions (see right). If the value of a coconut is not even constant for a single individual, we certainly cannot conclude that it will remain constant from one individual to the next.

Crusoe       first coconut       second coconut       third coconut       fourth coconut

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