214 ON THE CAUSES

But the most singular circumstance in this
section is, that it unsettles all our notions respecting
 quantity of labour itself. The grand
principle of Mr. Ricardo’s work, which seemed
as precise and definite as it could be, the doctrine
 that quantity of labour is the cause of
value, which appeared to be fast anchored in
the understanding, is unloosed from its moorings.
 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 quanlity
 of labour requisite to obtain commodities the principal
source of their exchangeable value.”