All index-number systems, so far as they are intended to have a greater significance for monetary theory than that of mere playing with figures, are based upon the idea of measuring the utility of a certain quantity of money. The object is to determine whether a gram of gold is more or less useful to-day than it was at a certain time in the past. As far as objective use-value is concerned, such an investigation may yield results. We may assume the fiction, if we like, that, say, a loaf of bread is always of the same utility in the objective sense, always comprises the same food value. It is not necessary for us to enter at all into whether this is permissible or not.

The Theory of Money and Credit