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

OR SCIENTIFIC METHOD. 2/3
the basis of the Method of Single Agreement it was asserted
that these minute living things were the immediate cause
of fermentation. But this was to go further than was
warranted by this method alone. It only gave a probability
that there is a causal connection. Accordingly, a countertheory,
supported by Liebig, held its ground for a consider
able time. He maintained that the connection between the
fermentive process and the living organisms is altogether
indirect ; that the yeast cells form a substance which by
purely chemical action produces the chemical change called
fermentation. Between these two theories the Method of
Single Agreement is powerless to decide. We shall see
what other methods were called in, by which in the end
the original hypothesis was established ( 7).
This method is applicable where our control over the
phenomena under investigation is very limited, so that
experiment, unless of an extremely rudimentary kind,
is not possible.
4. When the Method of Single Agreement has
suggested a causal connection and this, as we have
seen, is all that it can do an important means of
testing the supposition is provided
in the Method of
Difference (better named the Method of Single Differ
ence). This is essentially the Method of Experiment.
When we can produce the phenomenon ourselves, we
are not content with the mere general probability which
the Method of Agreement yields. We take the agent
believed to be the cause, and introduce it into definite
circumstances arranged by ourselves, where we know
therefore that whatever change follows must be due
to the agent which we have introduced. Sometimes
we add the agent to the known circumstances, at other
times we subtract it ; logically the results are the same.
Mill s statement of the canon is as follows :
"
If an
instance in which the phenomenon under investigation
occurs, and an instance in which it does not occur,
s