It must always be recollected, however, that in any sense in which we can use the term _value of a commodity_, there must be a reference, either expressed or implied, to some place and time, in the same manner as when we use the term _price of a commodity_. We all well know that the price of the same kind of commodity of the same quality, weight, and dimensions, is very different in different places and at different times; and this must be equally true in regard to the value of a commodity. It follows that, from the very nature of the thing, the value of a commodity cannot be expressed or measured independently of place and time. It is this quality which so essentially distinguishes the value of a commodity from its length or weight; but it does not necessarily destroy its capability of being measured. It is true, however, that a very general opinion has prevailed among political economists, even since the publication of Adam Smith’s work, that from the very nature of value, so essentially different from length or weight, it cannot admit of a regular and definite measure.[92] This opinion seems to me to have arisen principally from two causes. First—a proper distinction has seldom been made between the definitions of wealth and value. Though the meanings of these two terms have by no means always been considered as the same, yet the characteristics of one of them have been continually allowed to mix themselves with the characteristics of the other. This appears even in Adam Smith himself. When he says, that a man is rich or poor according to the quantity of the necessaries, conveniencies, and luxuries of life which he can command, he gives a most correct definition of wealth; but when he afterwards says, that he is rich or poor according to the quantity of labour which he can command, he evidently confounds wealth with value. The former is a definition of wealth; and of this, or of the general power of purchasing, which too much resembles it, there is no measure. The latter is his own definition or expression of real value; and of this the very terms which he uses show that there is a measure. The measure is distinctly expressed in the terms.