4. _There is a class of goods, natural agents and stores of materials which appears to be relatively fixed in quantity or which is increasable only with much difficulty._ The first part of this proposition expresses mildly the thought that long obtained among economists: it was said that the supply of certain things was absolutely fixed, the chief of these being land used for agriculture. The idea as held by Malthus and Ricardo was modified by John Stuart Mill in somewhat inconsistent ways. Land, it was said, is a thing which "man cannot make," therefore its supply is fixed. The second part of the opening proposition expresses the view here held: the supply of no important class of goods is absolutely fixed, in any reasonable sense. Most, if not all, belong to the class that is increasable, although it may be with much difficulty. Even when the exact thing cannot be duplicated, as a bust by an ancient sculptor or an autograph of a dead author, many substitutes serving the same or closely related wants, affect and limit the demand, and thus increase the supply. Men cannot, it is true, increase the stores of copper in the earth, but they devise new processes to extract it from ores before worthless, and invent methods of procuring aluminium, which yields some of the same utilities as copper. Even the supply of land, as is shown elsewhere, is constantly changing. Thus all kinds of wealth can be increased in some degree; many kinds in the course of time are very greatly increased with little or no direct effort, but the supply of all alike can be secured in larger amount at any given moment only at the cost of increasing difficulty. § II. SOCIAL SIGNIFICANCE OF THESE DIFFERENCES [Sidenote: Physical amount vs. economic supply] 1. _Not the fixity of the physical amount of agents, but the economic supply is significant._ There is danger of confusion between these two ideas. The statement that "land" cannot be created and that therefore "the supply is fixed" involves a fallacy. The word supply means the amount that is available at the moment or during the period spoken of. The land in Greenland is not, and probably never can be, a part of the supply of land in England. The land in America for centuries was not, but now has become, for some purposes, a part of the supply in the same market as the land of England. The question of importance in economic discussion is not whether the physical material can be brought into existence, but whether the economic "supply" can be increased. The existence of coal-mines in Venus or Mars is of no economic importance to us, but coal-mines on the earth, yet undiscovered, present a potential supply that at any moment may be realized. [Sidenote: Discovery enlarges the supply of natural resources]