it is an error in economics, as prevalent as it is patent, that all commodities, at a definite point of time and in a given market, may be assumed to stand to each other in a definite relation of exchange, in other words, may be mutually exchanged in definite quantities at will. . . . The most cursory observation of market phenomena teaches us that it does not lie within our power, when we have bought an article for a certain price, to sell it again forthwith at the same price.
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