“they themselves [_i. e._ demand and supply] obey a superior force, which makes value gravitate towards Cost of Production.... Demand and Supply always rush to an equilibrium, but the condition of stable equilibrium is when things exchange for each other according to their cost of production.”[157] Cost is called now by Ricardo’s words “natural value;” now by Malthus’s, “necessary price.” Cost of production is defined as follows, in the chapter which summarizes the theory of value: “Cost of production consists of several elements, some of which are constant and universal, others occasional. The universal elements of cost of production are the wages of the labour and the profits of the capital. The occasional elements are, taxes and any extra cost occasioned by a scarcity value of some of the requisites.” With respect to rent, Mill follows Ricardo for the general rule, instead of Malthus and Senior: “Rent is not an element in the cost of production of the commodity which yields it; except in the case (rather conceivable than actually existing) in which it results from, and represents, a scarcity value. But when land capable of yielding rent in agriculture is applied to some other purpose, the rent which it would have yielded is an element in the cost of production of the commodity which it is employed to produce.”[158] If Mill had followed the lead given in the last sentence to its logical end, the result ought to have been the inclusion of ground-rent as an element in entrepreneur’s costs. But, in spite of these concessions, he thought of rent habitually as failing to enter into price, and thus failing to be a cause of disproportionality between labor-cost and value. 2. The profit of capital is stated explicitly to be the remuneration of abstinence, but nothing is made to depend on this.[159] Abstinence is not elevated into a position logically coördinate with labor, nor are the two conceived of together as constituting subjective costs, as distinguished from entrepreneur’s costs, consisting in profits and wages.[160] While the language of Mill in diverse places shows that he was well enough aware of the difference between the two main forms of cost, so little was this essential distinction ever ready in his mind that he is able to say, without realizing the offence, just as Ricardo does: “Besides the natural and necessary elements in cost of production—labour and profits....”[161] When the time comes to develop the traditional philosophical account of value, abstinence is forgotten. Labor alone comes to the front. The aberration, due to profits, of the value of products from the standard required by labor cost, is treated not in a “philosophical” account like Senior’s, which endeavors to include and explain the case, but is regarded as an exception to the “philosophical” explanation, or as an error in it.