A brief historical sketch of the cost theory will be helpful. Costs are sometimes conceived as a cause of value, and sometimes as a measure of value. Often these two aspects are mixed, and writers shift from one notion to the other. This is particularly true of the labor theory. In Adam Smith the contention sometimes is that labor is unvarying in value, hence an admirable measure of values, and an excellent standard of long-time deferred payments. Smith compares wheat and silver from the standpoint of the constancy of their relation to labor, and concludes that wheat is the better standard in the long run, because it remains more nearly fixed with reference to labor than does silver. Sometimes Smith thinks of labor as a cause of value, and thinks of the labor that enters into the production of a good as the significant thing. At other times, the labor that goods will command or purchase is the significant thing--and here one is not clear whether he thinks of labor as a cause or as a measure. Whether labor is to be funded as labor-pain, or as labor-time, Smith does not state. Sometimes labor seems to be considered as homogeneous in its efficiency. At other times, he makes comparison between different kinds of labor as to their efficiency, and compares the efficiency of labor in different occupations. One can find nearly anything one pleases in Adam Smith on these points. At times he speaks of "labor and expense," rather than labor alone, as governing prices. Labor-cost to the laborer would take the form of labor-pain or labor-time. To the employer, it would take the form of outlay in wages. Adam Smith never makes any definite statement of point of view here, and shifts back and forth from one to the other. He recognizes variations in labor-pain, in danger, etc., in different kinds of labor when discussing wages.