Various attempts have been made to show why the shares in distribution are what they are, and not something else. Sometimes an attempt is made to establish a different regulating principle for each of the shares and to show that the amount of the product which goes as rent is fixed by one cause and the amount which goes as wages is fixed by another and independent cause, and so on for the four shares. At other times the effort has been made to show that there is one general cause working throughout the field of distribution, fixing and determining each of the shares. In the following pages the view will be maintained that there is one general principle according to which the division into the four shares is made. This general principle represents, of course, only-large general tendencies. There are many interfering causes at work which prevent the tendencies represented by this principle from being worked out in detail, but in a large way it does actually govern in the fixing of the shares in distribution. This governing principle is that of distribution according to marginal productivity. According to this general principle of distribution the possessor of a unit of any of the factors in production, namely, land, labor, capital, and enterprise, tends to get as income the marginal product of that unit.

168. What the principle of distribution according to marginal productivity does not mean. - When we say that the possessor of a unit of any factor in production tends to get for the use of that factor its marginal product, we do not mean that he gets exactly the marginal product. It will usually happen that there are countervailing forces which prevent this tendency from working out to a nicety. But the tendency is in general so strong that it may be considered the large principle according to which the division is made. When we say that the possessor of a unit tends to get its marginal product, we do not mean to say that he tends to get the actual product of some other unit than his own, for example, of some less efficient unit which is marginal in the sense that it would be the first unit to be thrown away if some unit were to be displaced. He tends to get the marginal product of his own unit of the factor. When we say that the owner of each unit tends to get the marginal product of that unit, we do not mean to imply that it is possible to take the physical products which have been made by the combined efforts of land and labor and capital and organizing skill, and to say just what parts of these physical products have been produced by a unit of land or by a unit of labor or by a unit of capital or by a unit of organizing ability.