The field of economic activity is the source of many problems. For, as Cassel says,[23] we include within the meaning of the word economic those means of satisfying human wants which are “usually available only in a limited quantity.” Since “the wants of civilized human beings as a whole are,” for all practical purposes, “unlimited,” there is in all economic life the constant necessity of reaching “an adjustment between the wants and the means of supplying the wants.” This disharmony of supply and demand is the source of an unending series of problems. We may note at once that the economist does not claim as his province the whole range of adjustments between human wants and the means of satisfying them. He usually omits, for example, the human need to breathe air. For since the air is unlimited in quantity the human need of it is not frustrated, and the surplus air not required by men in no way impinges upon their lives. Yet there may be a scarcity of air, as, for example, in a congested tenement district. Then an economic problem is engendered which has to be met, let us say, by building laws requiring a certain number of cubic feet of air a person. The economist, in other words, takes as his field of interest the maladjustment between human wants and those means of satisfying them which are available, but only in limited quantities. In a world where every want was satisfied there would be no problems for him; nor any in a world where men had no wants; nor any in a world where the only wants men had could be supplied by a change on their part of their own states of consciousness. To create a problem there must be at least two dependent but separated variables: wants and the means of satisfaction; and these two variables must have a disposition to alter so that an antecedent equilibrium is disturbed. In the measure, says Cassel, in which the economic system succeeds in securing an adjustment between the wants and the means of supplying the wants we speak of it as a sound economy. “This task may be accomplished in three different ways: first, by eliminating the less important wants and so restricting the total wants; secondly, by making the best possible use of the means available for the purposes in question; and, thirdly, by increased personal exertions.”[24] Since the problem arises out of the disharmony of supply and demand, its solution is to be found by increasing the supply or restricting the demand. The choice of method depends first of all on which it is possible in specific cases to follow, and, second, granting the possibility, on which is the easier or the preferred. Either method will give what we acknowledge as a solution. For when two variables are in an adjustment which does not frustrate the expectations of either there is no problem, and none will be felt to exist. FOOTNOTES: [18] T. R. Malthus, _An Essay on the Principle of Population_, Chapter II. [19] A. M. Carr-Saunders, _The Population Problem_, p. 28.