NOTES AND ILLUSTRATIONS. 249 
and repassing from one to the other, which has been the 
source of the mischief : one of these senses, and the only pro- 
per sense, is, ascertaining the mutual value of two commo- 
dities by their separate relations toa third ; the other is, as- 
certaining, when two commodities have varied in value, in 
which of them the variation has originated. The transition 
from one of these ideas to the other is, I think, perceptible 
in the doctrine examined in the text, that money is & good 
measure of value for commodities at the same time, but not 
for commodities at different times. In the first part of this 
proposition, the term measure is used in the former sense, 
and itis meant to assert, that the value of commaodities to each 
other is shown by their prices, or values in money. In 
the latter part of the proposition, a transition is made to 
the second meaning, and it is intended to say, that the 
value of a commodity in money at different periods does 
not show whether there has been any alteration in the cir- 
cumstances of its production ; whether any variation in its 
price has originated with it, or with the money in which 
its value is expressed. If we do not suppose this transi- 
tion to be made, but that one sense is rigidly adhered to, 
the proposition is liable to all the objections brought 
against it in the text. 
It is probably the latter construction of the term measure, 
ander which invariableness has been so generally supposed 
requisite. But this, as is shown in the course of the pre- 
sent chapter, would not be invariableness of value, but in-