Malthus, Thomas Robert. Definitions in political economy [ID D27229].
But if the term has been generally understood in this way, people must have been fully aware that value was essentially different from length: they would know perfectly well that a piece of cloth of a yard long would continue to be a yard long when it was sent to China; but that its value, that is, its general power of purchasing in China, or the estimation in which it was held there, would probably be essentially altered…
But still it appears to me that, for the reasons before stated, that is, because the labour of each place and time measures at that place and time the estimation in which a commodity is held, the state of its supply compared with the demand, the elementary costs of its production, the natural and necessary conditions of the supply, the proportion of the produce to the producers, &c. it must be considered as measuring, with a fair approach towards accuracy, the values of commodities at these places and times, so as to answer the question,—what was the value of broad-cloth of a certain description in the time of Edward III. in England? or, what is the value of money at present in China?...
When we speak of the value of silver in China, we cannot possibly mean the value of an ounce of Chinese silver brought to London, where, if it were pure, it would be precisely of the same value as an ounce of pure silver which had been in London from time immemorial. What alone we can correctly mean is, the estimation in which the ounce of silver is held in China, determined, at the time, by the state of the supply compared with the demand, and ordinarily by the quantity of Chinese labour and profits necessary to produce it; and if this be what we mean by the value of an ounce of silver in China, there can be no doubt that Chinese labour, and Chinese labour alone, can measure it. Even, however, if we mean the relation of an ounce of silver to all the commodities in China in succession, it would be impossible practically to form any approximation towards a just notion of the result, except by referring the silver to Chinese labour…