DEMAND 
35 
frequently, for the best of motives, choose 
to do what we do not like. 
It happens, however, that it is exceedingly 
inconvenient to speak of demand in terms 
of preferences, because bare preferences are 
expressible only in ratios, which are more 
awkward to handle than simple whole num 
bers. It is, therefore, customary to write 
of the utilities of things instead of the prefer 
ences for things, and to say, when one thing 
is preferred to another, that the thing preferred 
has greater utility than the other. For the 
sake of brevity I shall hereafter write in 
variably about utilities, but I shall understand 
by them merely the quantitative, objective 
expressions of preference ; and the student 
would act prudently if he got into the habit, 
on reading about utilities, of registering 
a mental note to the effect that they do not 
convey any particular doctrine with regard 
to the determination of human action, but are 
merely symbols standing for the facts of choice. 
We may now proceed to amplify and 
qualify the statement that the utility of 
things diminishes as we get additional supplies 
of them. We had better begin by carefully 
scrutinizing the management of income as a 
whole, and afterwards concentrate on a 
microscopic analysis of the relation between