Whereas estimative value is defined solely by the interaction of a need and an object and thus by a single interest in an isolated individual in appreciative value, as it now appears, there are two men who compare and there are four interests being compared; but the two private interests of each of the two contracting parties have first been compared with one another separately, and it is the results of this comparison that are then compared in order to arrive at an average estimative value; this equality of relation makes it possible to say, for example, that four measures of corn and five bundles of wood have an equal exchangeable value.