On the subject of foreign trade, it would show that its universally acknowledged effect in giving a stimulus to production, generally, is mainly owing to its increasing the value of the produce of a country’s labour by the extension of demand, before the value of its labour is increased by the increase of its quantity; and that the effect of every extension of demand, whether foreign or domestic, is always, as far as it goes, to increase the average rate of profits[N] till this increase is counteracted by a further accumulation of capital. On the subject of the accumulation of capital it would show that if the increase of capital be measured by the increase of its materials, such as corn, clothing, &c., then it is obvious that the supply of these materials may, by saving, increase so rapidly, compared with labour and the wants of the effective demanders, that with a greater quantity of materials the capitalist will neither have the power nor the will to set in motion the same quantity of labour, and that consequently the progress of wealth will be checked; but that if the increase of capital be measured, as it ought to be, by the increase of its power to command labour, then accumulation so limited cannot possibly go on too fast. On the general subject of demand and supply, it would show that they must be restored to their universal empire, both in reference to the prices of commodities, and the dependence of the progress of wealth on the due proportion maintained between them. If the cost of a commodity be considered as composed exclusively of the actual advances of the capital required for its production, which seems to be the most natural and correct mode of viewing it,[O] then it is obvious, that as both the prices and values of commodities are proportioned to these advances, with the _addition_ of profits very variable in their amount, neither of them can be determined by these advances alone, or by the costs of production so defined. We must therefore have recourse to demand and supply. And on the other hand, if profits be included in the costs of production, then, as it follows, from the constancy of the value of labour, that ordinary profits are determined by the ordinary demand compared with the ordinary supply of the products of the same quantity of labour, the certain conclusion must be, that demand and supply enter powerfully into the costs of production according to this latter definition, and that therefore their dominion as to prices and value is absolutely universal.[P]