This paper aims to examine Keynes’s choice of units in the General Theory,
in light of his philosophy of measurement established in A Treatise on
Probability. Departing from Russell’s absolutist conception of magnitude,
Keynes identifies three types of magnitude of probability. In this work
significant attention is paid to a concept of conventional measurement, which
is normic and arbitrary. This philosophy of measurement has developed later
in his economic writings such as “Index Numbers” and A Treatise on Money.
Contrary to the dominant interpretations on the “choice of units”, as suggested
by both the labor theory of value and the standpoint of reducing monetary
magnitudes to real magnitudes, it is found that Keynes employed the
conception of conventional measure.