VI Demand Price and Marginal Demand Price

§ 22. In the preceding discussion it has been tacitly assumed that the demand curve is also what may be called, by analogy with supply, a curve of marginal demand prices. This is not necessarily so. The marginal demand price of a quantity y of any commodity is the difference between the desiredness (as measured in money) to consumers in the aggregate of annual (or weekly) purchases of a quantity y and of a quantity (y + y) respectively. The demand price of y units is the price that maintains an annual (or weekly) purchase of a quantity y. Hence it is equal to the desiredness (as measured in money) of the least desired increment ( y) in a quantity y to the purchaser of that increment. If then the purchase of the marginal unit indirectly increases or diminishes the desiredness of their holdings to the purchasers of other units, the marginal demand price and the demand price will be different. For commodities the desire for which is partly a desire for the uncommon the curve of marginal demand prices, which, for a nil purchase, coincides with the demand curve, will fall further and further below it as purchases increase: for commodities the desire for which is partly a desire for the common the opposite of this is true; while for commodities which are desired solely on account of the direct satisfaction they confer, the two curves are identical. When the curves diverge, maximum satisfaction—the parties concerned being assumed to be similar in wealth and temperament—is attained with an output that equates marginal supply price, not with demand price, but with marginal demand price.