A further corollary touching capital may be pointed out. The gains derived from the handling of capital in the pecuniary respect have no immediate relation, stand in no necessary relation of proportion, to the productive effect compassed by the industrial use of the material means over which the undertaker may dispose; although the gains have a relation of dependence to the effects achieved in point of vendibility. But vendibility need not, even approximately, coincide with serviceability, except serviceability be construed in terms of marginal utility or some related conception, in which case the outcome is a tautology. Where, as in the case commonly assumed by economists as typical, the investing undertaker seeks his gain through the production and sale of some useful article, it is commonly also assumed that his effort is directed to the most economical production of as large and serviceable a product as may be, or at least it is assumed that such production is the outcome of his endeavors in the natural course of things. This account of the aim and outcome of business enterprise may be natural, but it does not describe the facts. The facts being, of course, that the undertaker in such a case seeks to produce economically as vendible a product as may be. In the common run vendibility depends in great part on the serviceability of the goods, but it depends also on several other circumstances; and to that highly variable, but nearly always considerable extent to which vendibility depends on other circumstances than the material serviceability of the goods, the pecuniary management of capital must be held not to serve the ends of production. Neither immediately, in his purely pecuniary traffic, nor indirectly, in the business guidance of industry through his pecuniary traffic, therefore, can the undertaker's dealings with his pecuniary capital be accounted a productive occupation, nor can the gains of capital be taken to mark or to measure the productivity due to the investment. The "cost of production" of goods in the case contemplated is to an appreciable, but indeterminable, extent a cost of production of vendibility--an outcome which is often of doubtful service to the body of consumers, and which often counts in the aggregate as waste. The material serviceability of the means employed in industry, that is to say the functioning of industrial capital in the service of the community at large, stands in no necessary or consistent relation to the gainfulness of capital in the pecuniary respect. Productivity can accordingly not be predicated of pecuniary capital. It follows that productivity theories of interest should be as difficult to maintain as productivity theories of the gains of the pecuniary employments, the two resting on the same grounds.