108

ON MEASURES

tainable variation during the process, is the necessary
 condition for obtaining that common
expression of the length of the two objects,
which will show their relation to each other.
But it is obvious that this relation of length
would be equally determined in whatever way
the common expression was obtained.
Now in the case of value, we obtain this
common expression without that physical operation
 here described. We learn the values of
two commodities in relation to the third, not
from the application of an instrument, first to
one commodity and then to the other, but from
intercourse with mankind, or from the inspection
 of documents in which they are registered.
 We equally obtain a common expression,
 but we obtain it by different means. But
the invariableness in the length of a measure of
space, as above described, is a circumstance belonging
 to the means employed to obtain a
common expression of length ; and as the means
of obtaining a common expression of value are
totally different, as in fact the common expres-