Sec. 5] INCOME SUMMATION 149 
their similarity. The same principle of equal and opposite 
services applies to both. When merchandise is changed 
from one warehouse to another, the first warehouse is 
credited with the change and the second debited. The 
warehouse which has rendered up the merchandise has 
done a service; that which has received it has done a 
disservice. A banker who takes money from his vault and 
puts it in his till will, if he keeps separate accounts for the 
two, credit the vault and debit the till. When wheat is 
imported from Canada, that nation is credited and the 
United States is debited with the value of the operation. 
We may, as in the case of continuous productive pro- 
cesses, divide up transportation districts by any arbitrary 
lines, and consider the passage of any articles across those 
lines as an interaction. 
§5 
The third class of interactions is the change of owner- 
ship of wealth or property. This has been called transfer. 
Transfers usually occur in pairs, and involve two objects 
transferred in opposite directions between two owners. 
This double transfer, we have called an exchange. Since 
an exchange consists of two transfers, and since a transfer 
is a species of interaction and as such is self-canceling, 
every exchange is self-canceling and cannot of itself con- 
tribute anything to the total income of society.! When a 
bookseller, for instance, sells a book, he credits his stock 
with the fact that it has brought in money, and the cus- 
tomer debits his library to the same amount. 
! The exchange does not duplicate income, but merely shuffles it 
about. It may and does put services of wealth where they are most 
needed, and thus results in a more effective use of income, just as 
credit and other forms of the divided ownership of wealth may make 
a more effective ownership of capital. In both these cases there is an 
increase of “total utility.” This needs to be considered in its proper 
place, but it must not stand in the way of our canceling the values 
of assets and liabilities or of services and disservices. These values, 
as is well known, are connected, not with total utilities, but with mar- 
ginal utilities.