When we say that the owner of a unit of one of the factors tends to get the marginal product of that unit, we think of the unit as a relatively small part of the whole supply of the factor. The smaller the unit in proportion to the whole supply, the more accurately will the tendency be realized. When we say that the owner of a unit of a factor tends to get its marginal product, we mean that he tends to get the difference between the total product under normal conditions of production and what is produced under conditions which are normal except for the fact that the one unit of the factor in question is not being employed in production. We mean then by the marginal product of this unit that this particular unit is just at the margin of being employed. Sometimes it is employed and at other times it is not employed, all of the other factors remaining the same. The marginal product is the amount which we impute to this unit of factor because it is produced when the unit is employed and it is not produced when the unit is not employed.