If Mr. Mill had always strictly adhered to that meaning of the term _demand for a commodity_ which signifies the quantity consumed, he might have maintained the position with which he heads the third section of his fourth chapter, namely, _that consumption is co-extensive with production_. This, however, is, in reality, no more than saying, that if commodities were produced in such abundance as to be sold at half their cost of production, they would still be somehow or other consumed—a truism equally obvious and futile. But Mr. Mill has used the term demand in such a way, that he cannot shelter himself under this truism. He observes, “It is evident that whatever a man has produced, and does not wish to keep for his own consumption, is a stock which he may give in exchange for other commodities. His will, therefore, to purchase, and his means of purchasing, in other words, his demand, is exactly equal to the amount of what he has produced, and does not mean to consume.”[17] Here it is evident that Mr. Mill uses the term demand in the sense of the amount of sacrifice which the purchaser is able to make, in order to obtain the commodity to be sold, or, as Mr. Mill correctly expresses it, his means of purchasing. But it is quite obvious that his means of purchasing other commodities are not proportioned to the _quantity_ of his own commodity which he has produced, and wishes to part with; but to its _value in exchange_; and unless the value of a commodity in exchange be proportioned to its quantity, it cannot be true that the demand and supply of every individual are always equal to one another. According to the acknowledged laws of demand and supply, an increased quantity will often lower the value of the whole, and actually diminish the means of purchasing other commodities. Mr. Mill asks, “What is it that is necessarily meant, when we say that the supply and the demand are accommodated to one another? It is this (he says) that goods which have been produced by a certain quantity of labour, exchange for goods which have been produced by an equal quantity of labour. Let this proposition be attended to, and all the rest is clear. Thus, if a pair of shoes is produced by an equal quantity of labour as a hat, so long as a hat exchanges for a pair of shoes, so long the supply and demand are accommodated to one another. If it should so happen that shoes fell in value, as compared with hats, which is the same thing as hats rising in value, as compared with shoes, this would imply that more shoes had been brought to market, as compared with hats. Shoes would then be in more than due abundance. Why? Because in them the produce of a certain quantity of labour would not exchange for the produce of an equal quantity. But for the very same reason, hats would be in less than due abundance, because the produce of a certain quantity of labour in them would exchange for the produce of more than an equal quantity in shoes.”[18]