5. _Wages and rent are coördinate species of the value problem; time-value is a different kind of problem, bearing to both the other problems a similar relation._ A close examination of the problems of rent and wages serves to bring out the close parallelism of these two forms of income as here defined. Rent is the value of the usufruct of wealth, wages are the value of the usufruct of labor. The bearer of the use in one case is material goods, in the other is human agents. Different in the source of use, they are in large measure alike in the form of contract, or nature of the calculation. Together rent and wages comprise the value of all currently arising uses; they are the two coördinate species of the genus "value of uses." The two groups of uses are closely interrelated in practice, each acting and reacting on the value of the other. Time-value is a different genus of the value problem. Having to do with time differences, it must be found in connection with every use that is not immediate, whatever be the bearer of that use. Its application to rent is more frequent and obvious, as only the uses of material agents are capitalized, that is, sold in perpetuity. Moreover any service of labor that is not at once consumed is fixed in material form and appears thenceforward as wealth whose uses are yielded as rent or as consumption goods. § III. THE RELATION OF LABOR TO VALUE [Sidenote: Several conditions of value] 1. _Labor is a cause, but only one of the causes of value._ A cause is some one condition which is seen to be necessary to the existence of a thing, and usually that condition which brings the thing about, other things being assumed. In what sense ought a cause of value be spoken of? In one sense it is in the minds of men--it is their wants; again, looked at objectively it is in the nature of the good--it is the quality that fits it to gratify the want. But if both these causes are operative, and labor is applied to fit goods better to gratify wants, labor appears as the cause of value. Personal causes are so much more evident, an explanation through personal causation is so much more satisfying in the earlier stages of scientific inquiry, that labor long continued to be looked upon as the one source of value. This erroneous view has never quite ceased to influence economic thought, and a great deal of effort has been directed to formulating theories of value based upon it. The cruder form of the error has now almost disappeared, but in various little recognized ways it still persists. [Sidenote: Two phases of economic production]