Getting more use out of the book by effort, out of the farm by applying more fertilizer, out of the house by putting more people into it, is intensive utilization. The earlier uses come easily, naturally; the later ones are gotten with increasing difficulty.
When a number of agents are of different qualities, the point between the one last used and the next unused is the extensive margin of utilization. The best agents that are available are naturally used first, but as they are more intensively used there is increasing inconvenience. Then recourse must be made to the inferior agents, whose first uses, however, are greater than the later, intensive uses, of the better grades. When the step is made to the use of agents that were before unused because inferior, it is extending the margin of utilization. The intensive margin of use is in the particular thing; the extensive margin of use lies outside of this.
Extensive Grades of Uses
The relation of the two margins may be shown in a simple diagram. Let the better grades of indirect agents be represented by longer rectangles, the upper parts of which represent the more accessible, more easily secured utilities. Each agent consists of many strata of uses. The best uses are grades a, b, and c, in M; but after M has been utilized intensively down to d, N will begin to be utilized at its highest point. When utilization goes down to f, O comes into use, and so on. Therefore it will be seen that until the intensive margin takes in d, M is on the extreme margin of utilization, and N is just outside it; when the intensive margin falls to g and h, P is inside the extensive margin, and Q is just outside.
Equilibrium of the two margins
The marginal utility or effectiveness of added agents tends to be equal on the intensive and the extensive margins. This is simply a case of the substitution of goods in the use of indirect agents. If the value of the added product in the use of a particular good decreases, a point finally is reached where it is better to transfer the outlay to another agent, to change from intensive to extensive utilization, to go over to the use of another field or of another machine not so good. The effectiveness of the labor or capital that men have to apply is being compared constantly in the two cases, and to the extent that this comparison is perfect the effectiveness of the agents tends to be equal on the margin in the two applications.
§ II. OTHER MEANINGS OF THE PHRASE "DIMINISHING RETURNS"
Does not mean declining prosperity