But we have a different and better description of value in the next section. It is there said, that “gold and silver are, in reality, commodities. They are commodities for the attainment of which labour and capital must be employed. It is cost of production which determines the value of these as of other ordinary productions.”[16] Now, if cost of production determines the value of money, it follows that, while the cost of producing a given quantity of money remains the same, its value remains the same. But it is obvious that the value of money may remain the same in this sense of the term, while, owing to the alterations which may be taking place in the costs of producing the commodities alluded to, the quantity of other things for which it will exchange may be essentially different. Which of the two, then, is the true criterion of the value of money? It is surely most desirable that the student in political economy should not be left in the dark on this subject; yet Mr. Mill gives him no assistance; and he is left to decide between two very different meanings as well as he can. But, perhaps, the most culpable confusion of terms which Mr. Mill has fallen into, is in relation to demand and supply; and as he has a more original and appropriate claim to this error than any other English writer, and its belief leads to very important consequences, the notice of it is particularly called for. In the first place, no person can have turned his attention, in the slightest degree, to the language of political economy, either in conversation or books, without being fully aware that the term demand is used in two very distinct senses; one implying the quantity of the commodity consumed, and the other the amount of sacrifice which the purchasers are willing to make in order to obtain a given portion of it. In the former sense, an increase of demand is but very uncertainly connected with an increase of value, or a further encouragement to production, as in general the greatest increase of such kind of demand takes place in consequence of a very abundant supply and a great fall in value. It is the other sense alone to which we refer, when we speak of the demand compared with the supply as determining the values and prices of commodities; and in this latter sense of the term demand, which, perhaps, is in the most frequent use, an increase of supply is so far from increasing demand that it diminishes it, while a diminution of demand increases it.