§ 7. Machinery thus figures as the efficient cause of industrial disease, but the real responsibility does not rest on the shoulders of the inventor of new machinery, or of the manufacturer, but of the consumer. The root-evil of depressed trade is under-consumption.[157] If a quantity of capital and labour is standing idle at the same time, in all or in the generality of trades, the only possible reason why they remain unemployed is that there is no present demand for the goods which by co-operation they are able to produce. English economists, most of whom, ever since the time of J.B. Say, have denied the possibility of the condition of general over-supply which is seen to exist in depressed trade, are contented to assume that there can be no general over-supply because every one who produces creates a corresponding power to consume. There cannot, it is maintained, be too much machinery or too much of any form of capital provided there exists labour to act with it; if this machinery, described as excessive, is set working, some one will have the power to consume whatever is produced, and since we know that human wants are insatiable, too much cannot be produced. This crude and superficial treatment, which found wide currency from the pages of Adam Smith and McCulloch, has been swallowed by later English economists, unfortunately without inquiring whether it was consistent with industrial facts. Since all commerce is ultimately resolvable into exchange of commodities for commodities, it is obvious that every increase of production signifies a corresponding increase of power to consume. Since there exists in every society a host of unsatisfied wants, it is equally certain that there exists a desire to consume everything that can be produced. But the fallacy involved in the supposition that over-supply is impossible consists in assuming that the power to consume and the desire to consume necessarily co-exist in the same persons.