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

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THE THEORY OF INDUCTION
of such magnitude as fermentation was caused by the
growth of an invisible vegetable organism ; hence
analogy suggested that the animalcuke, whose presence
was detected in the infected blood, might after all be
the true cause of the diseases in question. This
hypothesis, being experimentally verified, was proved
to be true by applications of the Double Method of
Difference. The old theory, that these diseases were
caused by a virus introduced into the blood, could
only give a forced explanation of many known facts ;
and it had to give way to a new theory, harmonising
all the facts. But the new theory was originally
suggested by analogy with the old ; and the specula
tions with regard to the action of the virus which were
based upon facts did not lose their value ; they simply
had to be revised by the aid of the new light shed on
the question.
13. We have seen that the perception of an analogy
may lead to the formulation of a general law as an hypo
thesis to explain two things between which some signi
ficant resemblance is discerned. We "two say things"
vaguely, rather than "two events," because one of them
may be itself an hypothesis or theory, and the other an
event which partly comes under it but suggests a modi
fication of it. We now proceed to consider the charac
teristics of a good hypothesis, and the conditions under
which we may regard an hypothesis as proved.
We must understand first that the invention of
hypotheses is the work of the scientific genius. In
the previous section we were discussing the ways in
which hypotheses might be "suggested"; but before
they can be suggested there must be a mind prepared
to receive the suggestion. They are the creations of
the investigator s mind. There is such a thing as