difference of result.

Mill's statement of this principle, which he calls the Canon of the
Method of Difference, is somewhat more abstract, but the proof relied
upon is substantially the same.

_If an instance in which the phenomenon under investigation
occurs, and an instance in which it does not occur, 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 effect, or][1] the cause, or an indispensable
part of the cause, of the phenomenon._

Mill's statement has the merit of exactness, but besides being too
abstract to be easy of application, the canon is apt to mislead in one
respect. The wording of it suggests that the two instances required
must be two separate sets of circumstances, such as may be put side
by side and compared, one exhibiting the phenomenon and the other
not. Now in practice it is commonly one set of circumstances that we
observe with a special circumstance introduced or withdrawn: the two
instances, the data of observation, are furnished by the scene before
and the scene after the experimental interference. In the case, for
example, of a man shot in the head and falling dead, death being the
phenomenon in question, the instance where it does not occur is the
man's condition before he received the wound, and the instance where
it does occur is his condition after, the single circumstance of
difference being the wound, a difference produced by the addition or
introduction of a new circumstance. Again, take the common coin and
feather experiment, contrived to show that the resistance of the air
is the cause of the feather's falling to the ground more slowly than
the coin. The phenomenon under investigation is the retardation of the
feather. When the two are dropped simultaneously 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. Then
the air is pumped out of the receiver, and the coin and the feather
being dropped at the same instant reach the ground together. This
is the instance where the phenomenon does not occur. The single
circumstances of difference is the presence of air in the former
instance, a difference produced by the subtraction of a circumstance.

Mill's Canon is framed so as to suit equally whether the significant
difference is produced by addition to or subtract