_The Law of Diminishing Productivity._--Under these conditions the second unit of labor will add something to the amount that was produced by the first unit, but it will not cause the product to become double what it was. It could not do that unless the capital also were doubled. Each unit of labor is now cooeperating with one half of the original capital, and the total product is less than it would have been if the new labor, on entering the field, had brought with it as full an equipment of productive instruments as was possessed by the labor that preceded it. Adding to the industry a second unit of labor without adding anything to the capital makes the total product somewhat larger, but falls short of doubling it. If we credit to this second unit of labor what it adds to the product that was created before it came into the field, we shall find that it is a certain positive amount, but obviously less than the total product which was realized by the first unit _and all the capital_. It is even less than a half of the product of the two units using all the capital. Perhaps the first unit of labor, when it used all the capital, created ten units of product; while the two units of labor, using this same original amount of capital, produce sixteen units of product. The clear addition to the original product which is caused by the added labor of the second squad of workmen is only six units, while a half of the total product after the addition to the labor has been made is eight. This figure represents the amount we may attribute to one unit of labor and a half of the total capital, while six represent what is _causally_ due to one unit of bare labor only. With all the capital and one unit of labor we get ten units of product, while the addition of one unit of bare labor brings the total amount up to sixteen. Six units find the cause of their existence in the presence of the second unit of labor, and the second unit therefore shows, as compared with the first, a diminished productivity. _Product of the Third Unit of Labor._--We will now introduce a third unit of labor, leaving the amount of capital still unchanged, but again altering the forms of it so as to adapt them to the needs of a still larger working force. We will make the buildings larger and therefore, of necessity, cheaper in their forms and materials. We will make the tools and machines more numerous and simple, and will do everything that is necessary in order to make the fixed amount of capital--the fund amounting to a given number of "dollars"--embody itself in the number and the kinds of capital goods that are requisite in order to supply three times the original number of workmen. The third unit of labor now adds something to the product realized by the first two, but the addition is smaller than it was in the case of the second unit.