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

OR SCIENTIFIC METHOD.
from the other. But several questions remain :
" How
are we to tell what the result would have been had the
season been wet instead of dry, or dry instead of wet ?
How are we to tell whether the manure is equally
useful for light soils and heavy, for gravels and marls
and clays?" The result is only established for the
particular circumstances of season and soil in which the
trial was made.
Hence the Method of Single Difference does not give
us a law of the action of the suspected cause e.g.,
that A will produce a under all circumstances ; unless
we pay special attention to the negative instance.
5. We have seen that, to apply the Method of
Single Difference successfully, we have to obtain the
most complete control of the negative instance. For
in order to establish the law that A is the cause of ,
we need to establish the two propositions :
Now, in order to prove
If A, then a ;
If not A, then not a.
the second of these two state
ments, it is usually necessary to conduct an independent
investigation into all the material negative instances.
We require to exhaust the field of negation by proving
that if A is absent, a is absent ; and this is far more
difficult than to exhaust the field of affirmation by
proving that if A is present, a is present. Thus the
essence of the new method that is required is to supple
ment the positive instances by examining the negative
instances. Now the positive instances may be taken
in accordance with the Method of Single Agreement
as well as the Method of Single Difference. Hence
there are two new methods :
(a) The Double Method of Agreement,
(b) The Double Method of Difference.