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 onProbability. Departing from Russell’s absolutist conception of magnitude,Keynes identifies three types of magnitude of probability. In this worksignificant attention is paid to a concept of conventional measurement, whichis normic and arbitrary. This philosophy of measurement has developed laterin his economic writings such as “Index Numbers” and A Treatise on Money.Contrary to the dominant interpretations on the “choice of units”, as suggestedby both the labor theory of value and the standpoint of reducing monetarymagnitudes to real magnitudes, it is found that Keynes employed theconception of conventional measure.