The proper mathematical analogy [between demand and supply] is that of an _equation_. If unequal at any moment, competition equalizes them, and the manner in which this is done is by an adjustment of the value. If the demand increases, the value rises; if the demand diminishes, the value falls; again, if the supply falls off, the value rises; and falls, if the supply is increased. The rise or the fall continues until the demand and supply are again equal to one another: and the value which a commodity will bring in any market is no other than the value which, in that market, gives a demand just sufficient to carry off the existing or expected supply. Mr. Cairnes(209) finally defined market value as the price “which is sufficient, and no more than sufficient, to carry the existing supply over, with such a surplus as circumstances may render advisable, to meet the new supplies forthcoming,” which is nothing more than a paraphrase of the words “existing or expected supply” just used by Mr. Mill. It seems unnecessary, therefore, that Mr. Cairnes should have added: “According to Mr. Mill, the _actual market price_ is the price which equalizes supply and demand in a given market; as I view the case, the ‘proper market price’ is the price which equalizes supply and demand, _not_ as existing in the particular market, but in the larger sense which I have assigned to the terms. To this price the _actual market price_ will, according to my view, approximate, in proportion to the intelligence and knowledge of the dealers.” Adam Smith, who introduced the expression “effectual demand,” employed it to denote the demand of those who are willing and able to give for the commodity what he calls its natural price—that is, the price which will enable it to be permanently produced and brought to market.(210) This, then, is the Law of Value, with respect to all commodities not susceptible of being multiplied at pleasure. § 4. Miscellaneous Cases falling under this Law. There are but few commodities which are naturally and necessarily limited in supply. But any commodity whatever may be artificially so. The monopolist can fix the value as high as he pleases, short of what the consumer either could not or would not pay; but he can only do so by limiting the supply. Monopoly value, therefore, does not depend on any peculiar principle, but is a mere variety of the ordinary case of demand and supply.