178 NATURE OF CAPITAL AND INCOME [Car X 
    
  
understand any one without having had at least some 
view of both of the others. 
    
  
  
  
  
  
  
    
   
  
  
  
  
  
    
  
   
   
   
  
   
   
   
§9 
Having completed our survey of the summation of the 
elements of income, we may properly pause to classify these 
items. They fall naturally into three groups. The first 
group includes those items of income which are positive 
and not negative, that is, the agreeable experiences of sub- 
jective income, for these, as we have seen, are the only 
final uncanceled positive items. The second group in- 
cludes items which are negative but not positive, namely, 
disagreeable psychical experiences, and consists of two 
classes: (1) the labor and trouble which are sacrificed for 
the sake of procuring income through objective channels, 
in other words, the toil of the producer; and (2) the dis- 
agreeable impressions produced in one’s consciousness by an 
abnormal state of the body, as aches, pains, and all sorts 
of illness, but which are not, like toil, voluntarily incurred 
for the sake of future return. The third group includes 
what we have called interactions, or items which are at 
once positive and negative, according to the point of view. 
Both of the first two groups are entirely subjective, and 
the last is entirely objective. The third group, interac- 
tions, constitutes by far the bulk of the items entering 
into income accounts, and includes all of those which 
enter into practical bookkeeping. It may be subdivided 
into two groups: (1) interactions outside of the human 
body, and (2) interactions between external wealth and 
the human body, or what have been called “final objec- 
tive services.” The following scheme shows further sub- 
divisions: —