The Theory of Money and Credit - Ludwig von Mises [44]
The doctrine of the unfruitfulness of money has another significance for our problem. It sheds light on the position of money within the class of things constituting private capital. Why do we include money in capital? Why is interest paid for sums of money on loan? How is it possible to use sums of money, even without lending them, so that they yield an income? There can be no doubt about the answers to these questions. Money is an acquisitive instrument only when it is exchanged for some other economic good. In this respect money may be compared with those consumption goods that form part of private capital only because they are not consumed by their owners themselves but are used for the acquisition of other goods or services by means of exchange. Money is no more acquisitive capital than these consumption goods are; the real acquisitive capital consists in the goods for which the money or the consumption goods are exchanged. Money that is lying "idle," that is, money that is not exchanged for other goods, is not a part of capital; it produces no fruit. Money is part of the private capital of an individual only if and so far as it constitutes a means by which the individual in question can obtain other capital goods.
3 Money Not a Part of Social Capital
By social or productive capital Böhm-Bawerk means the aggregate of the products intended for employment in further production. [19] If we accept the views expounded above, according to which money cannot be included among productive goods, then neither can it be included in social capital. It is true that Böhm-Bawerk includes it in social capital, as the majority of the economists that preceded him had done. This attitude follows logically from regarding money as a productive good; this is its only justification, and in endeavoring to show that money is not a productive good we have implied how baseless a justification it is.
In any case, perhaps we may suggest that those writers who include money among productive goods and consequently among capital goods are not very consistent. They usually reckon money as a part of social capital in that division of their systems where they deal with the concepts of money and capital, but certain obvious further conclusions are not drawn from this.