If, in our diagram above, we substitute for "social mind" the more general expression, "human factor," we should find that our value concept is the common property of many writers. We should find it fitting in with the absolute value notion of Adam Smith and of Ricardo.[11] The "human factor" which _explains_ the absolute value is, for them, labor. We should find it fitting in with the "socially necessary labor time" of Marx: the value of a bushel of wheat is the amount of labor time which, on the _average_, is required to produce a bushel of wheat. It is an absolute value. It is a causal coefficient with the absolute value, similarly explained, of the bushel of corn, in explaining the wheat-price of corn. Our concept will fit in exactly with the "social use-value" of Carl Knies, according to whom the economic value of a good in society is an _average_ of its varying use-values to different individuals in the market. This average is an absolute quantity. The absolute values of units of two goods, thus explained, causally fix the exchange ratio between the goods. Knies' value-theory, it may be noticed, is explicitly modeled on that of Marx, to whom he refers, the difference being that Knies takes an average of individual use-values, while Marx takes an average of individual labor-times, as the causal explanation.[12] Our value concept will fit perfectly with Professor J. B. Clark's "social marginal utility" theory of value. Indeed, the present writer gratefully acknowledges that the concept is Professor Clark's rather than his own, and that all that is necessary for its explanation has been set forth by Professor Clark.[13] Professor Clark's _causal_ theory of value, his explanation of this absolute quantity of value as a _sum_ of individual marginal utilities, we have elsewhere[14] criticised as involving circular reasoning, like all marginal utility theories, in so far as they offer causal explanations. But his statement of the logical character of value, of the relation of value to wealth, of value to price, of value to exchange, of the functions of the value concept in economic theory, and of the functions of value in economic life,--Clark's doctrines on these points we have accepted bodily, and in so far as the present writer has added anything to them it has been by way of elaboration and defence.