214 ON THE CAUSES 
But the most singular circumstance in this 
section is, that it unsettles all our notions re- 
specting quantity of labour itself. The grand 
principle of Mr. Ricardo’s work, which seemed 
as precise and definite as it could be, the doc- 
trine that quantity of labour is the cause of 
value, which appeared to be fast anchored in 
the understanding, is unloosed from its moor- 
ings. We are here told of “the difficulty of 
comparing an hour’s or a day’s labour in one 
employment with the same duration of labour 
in another.” The language of Adam Smith is 
quoted, to show “ that it is often difficult to 
ascertain the proportion between two different 
quantities of labour. The time spent in two 
different sorts of work will not always determine 
this proportion.” 
If this be true, then quantity of labour has no 
determinate criterion, and Mr. Ricardo has 
perly qualified, is the Index. He there says, ‘ the quan- 
lity of labour requisite to obtain commodities the principal 
source of their exchangeable value.”