5. _Monopoly is such a degree of control over the supply of goods in a given market that a net gain will result to the seller if a portion is withheld._ Every producer has control over some agents and some portion of the supply of products; but ordinarily the portion controlled by any one is so small that withholding it entirely from sale would not cause the market price to rise in any appreciable degree. The producer in such a case regulates his action as if the market price were fixed beyond his control, and he uses his productive agents fully up to the point where costs equal price on the marginal unit of product. A skilled worker getting five dollars a day loses that sum every day he is idle. A landowner whose land can command a competitive rent of ten dollars an acre must take that sum or less, or nothing; he cannot get more. How can a net gain ever result from a smaller sale? As a reduction of supply results in a higher price, it is possible, as is seen in the paradox of value, for a situation to arise in the case of some goods, where a smaller number of units yield a larger sum in the market than a larger number of units. But the seller's interest lies not in the increase of total sales, but in that of net gains. Net gains, being the product of the number of units sold multiplied by the gain on each unit, increase at a much faster rate than do total sales. The existence of monopoly power in any degree depends therefore on several factors: the effect of contraction of supply in raising prices, the effect on costs, the number of units remaining in the ownership of the one contracting supply, and the possibility of preventing others from increasing supply later to profit by the higher prices. § II. KINDS OF MONOPOLY [Sidenote: The sources of monopoly power] [Sidenote: Political monopoly]