But when barter ceases, and money has become the common instrument of commerce, every particular commodity is more frequently exchanged for money than for any other commodity. The butcher seldom carries his beef or his mutton to the baker or the brewer, in order to exchange them for bread or for beer; but he carries them to the market, where he exchanges them for money, and afterwards exchanges that money for bread and for beer. The quantity of money which he gets for them regulates, too, the quantity of bread and beer which he can afterwards purchase. It is more natural and obvious to him, therefore, to estimate their value by the quantity of money, the commodity for which he immediately exchanges them, than by that of bread and beer, the commodities for which he can exchange them only by the intervention of another commodity; and rather to say that his butcher's meat is worth threepence or fourpence a-pound, than that it is worth three or four pounds of bread, or three or four quarts of small beer. Hence it comes to pass, that the exchangeable value of every commodity is more frequently estimated by the quantity of money, than by the quantity either of labour or of any other commodity which can be had in exchange for it. Gold and silver, however, like every other commodity, vary in their value; are sometimes cheaper and sometimes dearer, sometimes of easier and sometimes of more difficult purchase. The quantity of labour which any particular quantity of them can purchase or command, or the quantity of other goods which it will exchange for, depends always upon the fertility or barrenness of the mines which happen to be known about the time when such exchanges are made. The discovery of the abundant mines of America, reduced, in the sixteenth century, the value of gold and silver in Europe to about a third of what it had been before. As it cost less labour to bring those metals from the mine to the market, so, when they were brought thither, they could purchase or command less labour; and this revolution in their value, though perhaps the greatest, is by no means the only one of which history gives some account. But as a measure of quantity, such as the natural foot, fathom, or handful, which is continually varying in its own quantity, can never be an accurate measure of the quantity of other things; so a commodity which is itself continually varying in its own value, can never be an accurate measure of the value of other commodities. Equal quantities of labour, at all times and places, may be said to be of equal value to the labourer. In his ordinary state of health, strength, and spirits; in the ordinary degree of his skill and dexterity, he must always lay down the same portion of his ease, his liberty, and his happiness. The price which he pays must always be the same, whatever may be the quantity of goods which he receives in return for it. Of these, indeed, it may sometimes