Skc. 8] INCOME SUMMATION 145 
Interactions are essentially identical with what were dis- 
cussed a generation ago under the title “productive serv- 
ices.” But inasmuch as the name “productive services” 
is not a very happy one, and its use has been so confused and 
has engendered so many verbal quibbles, it seems advisable 
not to revive it. The essential fact that these “productive 
services” were two-faced — negative as well as positive — 
was always overlooked, and there remained no other charac- 
teristic which could give the phrase a definite and scientific 
meaning. 
Interactions constitute the great majority of the elements 
which enter into income and outgo accounts. The only 
services which are not merely the positive side of interac- 
tions are mental satisfactions — desirable conscious experi- 
ences — often miscalled “consumption”; and the only 
disservices which are not the negative side of interactions 
are pains or “labor.” But these are only the outer fringes 
of the economic fabric. Between them is a connective net- 
work of productive processes and commercial transactions, 
every fiber of which has two sides, a positive side of serv- 
ices and a negative side of disservices. 
$3 
The interactions between two articles or groups of articles 
may, of course, consist either in causing or preventing 
changes or events. The events or changes which are 
caused or prevented are of three chief kinds, — changes of 
form of wealth, changes of position, and changes of owner- 
ship; or, transformation, transportation, and transfer. We 
shall take these up in order. 
What is here called transformation of wealth is prac- 
tically identical with what is usually understood by “pro- 
duction” or “productive processes.” * By the transforma- 
tion of wealth, or the changes produced in its form, is 
meant the changes of relative position of its parts. Weav- 
1 Cf. Marshall, Principles of Economics, 3d ed., p. 132. 
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