OF VALUE.

109

sion is necessarily implied in the supposition
of using any commodity as a medium of com-
parison, there is nothing in the latter case in
which invariableness of any kind, or in any
sense, can be required. In the one case there is
an instrument employed in a physical opera-
tion, and it is for the purpose of rendering this
instrument capable of performing its function,
that invariableness is indispensably necessary :
in the other case there is no instrument so em-
ployed, and therefore there is no invariableness
wanted: in the former case invariableness in
the instrument (under the modification which it
is needless to repeat) is essential to the attain-
ment of the common term; in the latter, the
common term being given, there is nothing in
which invariableness can have place, or of
which it can be predicated. If the length of
the rod varied in an unknown degree between
applying it to the two objects, we should have
two terms of unknown relation to each other,
and there could be no comparison of the ob-
jects to be measured ; and if the values of the