144 NATURE OF CAPITAL AND INCOME [Crar. IX
promising to compensate for them by better service later
on. The renailing of loose shingles is certainly not what
the house is for, but is only a necessary evil. On the part
of the hammer, however, these same events are services.
The service called “nailing” is credited to the hammer.
Therefore the repairing of the house is at once a service
and a disservice.
Such double-faced events require a special name. We
may christen them interactions between two instruments
or groups of instruments. Alternative names are inter-
acting services, intermediate, or preparatory services,
coupled services, or simply “couples.” They underlie
what business men call “double entry bookkeeping.”
An interaction, then, is a service of the acting instrument,
a disservice of the instrument acted on. There can never
arise the slightest doubt as to when it is to be regarded as
positive and when negative. The definitions of service and
disservice settle this question in each case, by referring it
to the desire of a human being, viz. the owner of the service
or disservice. As he desires that the house should not oceca-
sion repairs, these repairs are disservices of the house: as
he desires that the tools should occasion repairs, they are
services of those tools. The hammer exists for and derives
its value from its prospective services in renailing shingles.
The house does not exist for nor derive its value from the
renailing of its shingles; on the contrary, the prospect of
that event detracts from its value.
The example given is typical of the general relations
between interacting instruments. The mental picture we
should construct is that of two distinet groups of capital.
Group A acts on group B for the benefit of the latter.
Whatever the nature of this interaction, 4 is credited with it
and B debited. The credit and debit are equal and simul-
taneous, the only result of the interaction being that, in
consequence of it, B is enabled at some later time to yield
more income.
