DEMAND 
55 
surplus got from a thing, regarded as a 
saving of a part of what would have been 
paid for the thing had the payment of its full 
value been unavoidable, is spent on other 
things and so becomes merged in real income, 
that is income not in money but in goods. And 
it is true, admittedly, that we must not 
thoughtlessly add up the surpluses got by an 
individual from different things and call the 
sum his total consumer’s surplus. Take 
away one article from a person and his 
demand for certain other articles will be 
increased, while his demand for certain 
other articles will be decreased. Some 
articles are so closely related in demand that 
their utilities are commingled—and in some 
degree the utilities of all things are com 
mingled. One thing helps to create the utility 
of another thing, as milk does that of tea ; in 
which case they are said to be complements. 
Again one thing undermines the utility of 
another thing, as tea does that of coffee—- 
without tea we should prize coffee more highly 
—in which case they are called substitutes. 
Nevertheless where it is possible to make 
due allowance for the actions and reactions 
of things on one another’s value and get at 
the aggregate consumer’s surplus with refer 
ence to some individual there would be a