The key to Marx’s economic doctrines is his theory of value, with an exposition of which “Das Kapital” opens. It is based on Ricardo and Rodbertus, but is developed and defended in an original manner. He begins by separating value in use from value in exchange. Value in use is utility, arising from the adaptation of an article to satisfy some human need. Air, water, sunshine, wheat, potatoes, gold, and diamonds are examples. It does not necessarily imply exchange value. Many goods are very useful but not exchangeable, because they are free to all. Such is the case, usually, with water. On the other hand, no good can have value in exchange unless it is useful. Men will not give something for that which satisfies no want or need. Both value in use and value in exchange are utilities, but, as they differ, there must be some element in the one which the other does not _per se_ contain. We find what that is by analyzing the constituent elements of different goods which possess exchange value. How can we compare them? Only because they contain some common element. But what is there in common between a horse and a house? You cannot say that this stick is longer than that sugar is sweet. Yet you say this house is worth ten times as much as that horse. Materials are not compared, nor stability with swiftness, nor color with color. The common element is found alone in human labor. You compare labor with labor. It requires ten times the amount of average social labor (_gesellschaftliche Durchschnittsarbeitskraft_) to secure such a house as it does to put one in possession of such a horse. Labor-time is the measure which we apply to different commodities in order to compare them. We mean thereby the ordinary average labor which is required at a given time in a given society. The average man is taken as a basis, together with the average advantages of machinery and the arts. This is average social labor-time. Complicated labor is simply a multiple of simple labor. One man’s labor, which has required long and careful training, may count for twice as much as ordinary, simple labor; but the simple labor is the unit. This distinction between value in use and value in exchange enables us to understand how capitalists exploit their laborers. They pay for labor its exchange value, which depends upon the cost of labor or the standard of life of the laborer, as we have already seen in our examination of Rodbertus’s system. What it takes to support a laborer’s family is the exchange value of all the labor which can be got out of that family. Let us suppose that a laborer requires each day goods whose value is denoted by A, each week in addition thereto goods denoted by B, besides quarterly needs which are satisfied by goods whose value is C. Then his support for each day will require the value of 365 A + 52 B + 4 C[176] ------------------ 365.