An introductory text-book of logic - Mellone, Sydney - Rare Books at ...

274
THE THEORY OF INDUCTION
have every circumstance in common save one, that
one occurring only in the former ; the circumstance
in which alone the two instances differ is the cause or
an indispensable part of the cause of the phenomenon."
The canon may be more simply and as follows :
clearly expressed
When the addition of an agent is followed
by the appearance, or its subtraction by the disap
pearance, of a certain event, other circumstances
remaining the same, that agent is the cause of the
event. When the suspected agent is present, we have
the positive instance ; when it is absent, the negative
instance. What cannot be eliminated without doing
away with the event, is causally
connected with it.
One of the simplest illustrations of this method is seen in
the coin and feather experiment, designed to show that the
resistance of the air is the cause of a light article, as a feather,
falling to the ground more slowly than a heavier one, as
a coin. The phenomenon to be investigated is the retard
"
ation of the feather. When the two are dropped simul
taneously in the receiver of an air-pump, the air being left in,
the feather flutters to the ground after the coin. This is the
instance where the phenomenon occurs (the positive instance).
Then the air is pumped out of the receiver, and the coin and
feather, being dropped at the same instant, reach the ground
together. This is the instance where the phenomenon does
not occur (the negative instance)." The single circumstance
of difference is the presence of the air in the former case,
and with its removal the retardation of the feather s fall
is removed.
In further illustration of this method, we may return to the
first of the examples given in the previous section. The pro
duction of colours by light passing through spherical and
prismatic glasses had already been noticed ; and
Newton
proceeded to make it the subject of exact experiment by
repeated applications of the Method of Single Difference.
A beam of the sun s light admitted through a small hole in
an otherwise darkened room, produces on a screen a circular