It has been objected to commerce that it is #sterile# and produces no new goods. There exist neither more nor less coal and wine after they are exchanged than before. But in political economy we treat of utility and wealth; the question is whether things are usefully consumed or not. Now that which is not wealth if it were consumed by one person, becomes wealth when handed over to another person for consumption. #Though exchange cannot create the material of wealth, it creates wealth because it gives utility to the material.# #72. Value means Proportion in Exchange.# When we speak of the value of a thing in exchange, we mean how much of some other thing we can get for it. This of course will depend upon the nature of that other thing. Obviously, I can get for a shilling much more potatoes than bread, and bread than beef, and beef than essence of beef. Therefore, when we speak of the value of a thing, we ought always to say what it is to be valued by. #The word value only means that so much of one thing is given for so much of the other#, and it is the #proportion# of these quantities (Latin _proportio_ from _pro_, in comparison with, and _portio_, share), which measures the values of the thing. A ton of pig-iron can usually be got for a quarter of corn; here the proportion is one to one. To get a ton of copper, we should probably have to give thirty quarters of corn; here the proportion is that of one to thirty. There cannot be such a thing as value in exchange, unless there be proportion--so much of one commodity for so much of another. Usually, indeed, we measure the values of things by their #prices#. The #price is the quantity of money which we give for a thing#; in this case the proportion is between the quantity of money and the quantity of goods we get for it, as when we give sixty shillings for ten yards of carpet. We shall learn later on that money is a kind of commodity, which has utility and value like other commodities. But there is great convenience in always thinking and speaking of values in money, because we can then readily compare the value of one thing with that of any other. If a pound of potatoes costs one penny, a pound of bread threepence, and a pound of beef ninepence, we can see at once that a pound of beef is of the same value as three pounds of bread and nine pounds of potatoes, and we can judge how much of each to use.