AT DIFFERENT PERIODS. 9] 
surd; but I lie under a great mistake, if it is 
not really the proposition which X Y Z* has 
been attacking, while he supposed himself to 
be in logical combat with the first. 
It is difficult to imagine how an error of this 
kind (if I am right in supposing it to be one) 
should have escaped a mind evidently well 
versed in the detection of ambiguities in argu- 
mentation. It appears to have arisen, as in the 
case of Mr. Ricardo, from blending the compa- 
rison of contemporary commodities with that 
of the same commodity at different periods, 
which led them to the erroneous inference, that 
because the valueof A at one period did not bear 
the same relation to the value of its producing 
labour as at another period, therefore the values 
of two contemporary commodities did not bear 
the same relation to each other as the values of 
the labour respectively bestowed on their pro- 
duction. 
* X Y Z is the designation assumed by the Author of the 
Dialogues.