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

THE GENERAL NATURE OF INDUCTION. 245
combine." This requires also a temperature free from ex
tremes of heat and cold much greater than those found on
the earth. Now some of the planets may resemble the
earth in all the ways enumerated by Reid, and yet may
not provide for these strictly essential conditions.
We have seen that Analogy may be expressed as an in
conclusive but probable argument in fig. ii., an enthymeme
from a "sign." Hence, as we may have a convergence of
"
signs," so we may have a convergence of analogical argu
ments, leading to practical certainty ; thus :
(a} In districts of the earth now exposed to glacial action
"
"
we find scored or striated rocks ;
In such and such a valley in Great Britain we find
striated rocks ;
Therefore this valley probably has been exposed to
glacial action.
(b) In districts now exposed to glacial
action we find
perched boulders ;
In the same valley we find perched boulders ;
Therefore this valley has been exposed to glacial
action.
(c) In districts now exposed to glacial action we find
lateral and terminal "moraines";
In the same valley we find lateral and terminal
moraines ;
Therefore this valley has been exposed to glacial
action.
Such a convergence of analogies, each inconclusive if taken
by itself, leaves no room for doubt. Of one such case,
Darwin said :
" A
house burnt down by
1
its story more plainly than did this valley."
fire did not tell
5. In his discussion of the Enthymeme and Para-
methods of inductive
deigma, Aristotle has analysed
1 The student should notice that the English word analogy comes
from the Greek ai>a\oyia, but has changed the meaning of its Greek
original. The word avaXoyia is used for what we call proportion in
mathematics, an equivalence of ratios, laoT-rjs \6ywv. In this sense
of the word it would be said that the relation of four to two is anal
ogous to that of six to three.