3. _When two agents are necessary to secure a product, the value attributed to each is influenced by competing uses._ The thought of one agent independently producing a certain product is far too simple to correspond with reality. Two or more agents unite to produce a single product, and each agent at the same time can be used for acquiring other products. Complex as the problem appears, it is solved according to the principle of marginal utility at every moment in every market. The different uses, figuratively speaking, bid for an agent, and thus its marginal utility is determined just as is the price of a good by the bidding of buyers. Indeed, it is the bidding of buyers, indirectly. The more urgent the use, the higher the bid. The felt importance is reflected from the consumption goods that are sought, to the agent that will aid to get them. Two or more agents that are mutually needed for the acquiring of a product are complementary goods. A complementary agent may be either other material agents or labor. [Sidenote: Complementary agents used intensively show diminishing returns] When labor is applied to an agent, either to improve the Quality or to increase the quantity, it is subject to the law of diminishing returns. In the effort to increase the quantity of products, labor is applied first more intensively to the better agents. If it meets with resistance, if returns diminish, it is transferred to any of the poorer agents that have in them uses of as high grade as those still in the better agent. The superior effectiveness of the earlier over the later units of the added agent is called the "differential advantage" of the two fixed agents. The result of a day's labor applied to a field may be represented by 100, a second day's labor by 90 (it being only ninety per cent, as effectual), a third day's labor by 75; but it is more usual to say that the first field produces 10 more than the second and 25 more than the third, the second 15 more than the third. To the agent fixed in supply is attributed the difference in the effectiveness of the agent that is applied. [Sidenote: The relentless extensive margin of agents] 4. _The marginal uses of indirect goods are free uses._ Here again is noted the close parallelism in the process of evaluating direct and indirect goods. There is an extensive margin in the use of an indirect agent, a point in the gradation from the better to the poorer agents where the materials and forces are left unused and have no value. Land beyond that point is free. Outworn goods in manifold forms, old pictures, old machines, having no longer charms even for a rummage sale, form a no-rent margin of wealth. On every hand a great multitude of things unused and worthless differ by only a shade from things that still are used and valued. Every rubbish-heap, rag-bag, junk-shop, and garret contains things once prized, now lingering on the margin of utilization.