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

278
THE THEORY OF INDUCTION
We shall see that the second of these is of more im
portance than the first. The Double Method of
Difference is indeed the fundamental method of
science ; other methods are only imperfect approxima
tions to it. Mill and his followers (e.g., Fowler) fail
to distinguish the two methods, or to show the import
ance of the Double Method of Difference. Its true
character is indicated by Laurie (/..), and (under
different names) by Bosanquet, Lotze, and others.
Mill s third Method is called by him the "Joint
"
Method of Agreement and Difference or the
"
Indirect
Method of Difference." Most logicians, however, describe
it more simply as a
"
Double Method of Agreement,"
agreement in presence combined with agreement in
absence.
6. The Double Method of Agreement is stated by
"
If two or more instances
Mill in the following canon :
in which the phenomenon occurs have only one circum
stance in common, while two or more instances in which
it does not occur have nothing in common save the
absence of that circumstance, the circumstance in which
alone the two sets of instances differ is the cause or the
effect or an indispensable part of the cause of the
phenomenon." This is vague, and it is incorrect in
more than one point : two positive instances would
never be enough, still less could two negative instances ;
and it is not necessary that the negative instances
should have
"
nothing in common." The following
Whatever is present
simpler canon may be proposed :
in numerous observed instances of the presence of
the phenomenon, and absent in observed instances
of its absence, is probably connected causally with
the phenomenon. This method presupposes that we
have had a wide and varied experience of the conjunc-